ELECTRICITY  IN 
MINING 


I 


The  D.  Van  NoStrand  Company 

intend  this  book  to  be  sold  to  the  Public 
at  the  advertised  price,  and  supply  it  to 
the  Trade  on  terms  which  will  not  allow 
of  reduction. 


ELECTRICITY     IN     MINING 


ELECTRICITY     IN 
MINING 


BY 


SYDNEY    F.    WALKER 

M.I.E.E.,  M.I.MiN.E.,  Assoc.M.I.C.E.,  &c. 


NEW   YORK 
D.   VAN   NOSTRAND   COMPANY 

23    MURRAY   AND    27    WARREN    STREETS 
1907 


PREFACE 


THE  Author  has  been  engaged  during  the  last  thirty  years  in  the 
practical  application  of  electricity  to  mining  work.  For  a  long 
period  he  was  the  apostle  of  electricity  to  the  mining  world,  preach- 
ing the  advantages  of  electricity  for  mining,  in  season  and  out  of 
season.  For  a  large  portion  of  the  thirty  years,  he  conducted  a  very 
uphill  battle  against  the  natural  conservatism  of  mining  engineers 
and  mine  managers,  who  have  to  think  before  all  things  of  getting 
their  mineral  out,  and  who  could  not  afford  to  instal  new  apparatus, 
however  good  it  might  seem,  unless  they  were  quite  sure  that  it 
would  not  interfere,  even  temporarily,  with  the  output ;  and  against 
the  many  difficulties  inherent  in  the  development  of  a  new  industry. 
He  claims  to  have  had  a  very  large  share  in  making  the  position  held 
by  electricity  in  mining  work  at  the  present  time ;  and  in  the  following 
pages  he  has  endeavoured  to  give  mining  engineers,  mine  managers, 
and  all  who  are  interested  in  mining  work,  or  who  have  to  do  with 
mines,  the  full  benefit  of  the  experience  acquired,  often  very  pain- 
fully, during  the  last  thirty  years.  He  has  also  endeavoured  to  give 
as  full  particulars,  and  to  explain  the  working  as  fully  as  possible,  of 
the  very  latest  up-to-date  apparatus  employed  both  in  this  country, 
in  America,  and  on  the  Continent. 

In  Chapter  I.  he  has  given  the  usual  resum6,  made  as  full  as 
possible,  and  to  meet  the  greater  knowledge  of  the  subject  possessed 
at  the  present  day  by  mining  engineers,  than  was  possessed  thirty 
years  ago,  of  the  underlying  principles  of  electricity,  with  the  terms, 
etc.,  in  general  use.  In  Chapters  II.  and  III.  he  has  given  short 
descriptions  of  signals,  telephones,  and  electric-lighting  apparatus 
in  use  about  mines,  that  in  his  opinion  can  be  employed  with 
advantage.  In  Chapter  IV.  he  has  gone  very  fully  indeed  into  the 
question  of  the  generation  of  electricity  economically.  It  has  ap- 
peared to  him  that  if  electricity  is  to  be,  as  he  believes  it  must  be, 

v  I 


vi  PREFACE 

the  agent  employed  about  mines  for  distributing  energy,  it  is  abso- 
lutely necessary  that  the  electricity  itself  should  be  generated  in  the 
most  economical  way  possible,  and  he  has  therefore  discussed  every 
possible  source  of  power  that  may  be  available,  and  every  possible 
source  of  economy. 

In  Chapter  V.  he  has  discussed  the  principles  and  practice  of  the 
distribution  of  electricity,  as  it  is  applicable  to  mining  work,  giving 
descriptions  of  every  method  of  distribution,  even  some  of  those  that 
in  his  opinion  will  not  often  be  applied,  because  it  has  come  to  his 
knowledge  that  they  have  been  applied  in  a  few  cases. 

In  Chapter  VI.  he  has  dealt  very  fully  with  the  application 
of  electricity  to  the  different  machinery  about  a  mine.  In  writing 
this  chapter,  the  Author  has  had  before  his  mind  the  fact  that  there 
are  a  large  and  increasing  number  of  engineers  who  are  engaged  in 
the  application  of  electricity  to  mining  work,  and  who  necessarily  are 
not  familiar  with  the  working  of  mines,  when  they  first  come  to  them. 
In  the  past  there  have  been  a  very  large  number  of  failures,  and  a 
very  large  amount  of  money  wasted,  owing  to  the  fact  that  those  who 
have  been  engaged  in  applying  electricity  to  some  mining  problem, 
have  not  been  acquainted  with  mines,  and  have  been  far  too  sanguine 
as  to  the  amount  of  power  required  for  performing  a  given  amount  of 
work.  Pathetic  complaints  have  been  made  at  the  Mining  Institute, 
by  those  who  have  been  advising  mining  engineers  in  the  application 
of  electricity,  that  motors  supposed  to  deliver  a  great  many  horse- 
power would  not  perform  the  work,  say  in  hauling  trams,  that  was 
easily  performed  by  one  horse  in  the  flesh.  It  has  been  the  Author's 
endeavour,  in  writing  Chapter  VI.,  to  provide  information  that  will 
enable  whoever  may  be  engaged  in  mining  work,  to  form  a  very  safe 
idea  of  the  power  he  must  provide  in  each  case,  while  on  the  other 
hand  he  has  endeavoured  to  show  mining  engineers  and  mine 
managers  how  that  power  is  to  be  delivered,  and  how  they  are  to  know 
when  they  have  the  proper  amount. 

In  Chapter  VII.  the  Author  has  given  a  few  simple  rules  for  the 
discovery  of  "  Faults,"  or  causes  of  failure.  In  this  chapter  also  he 
has  had  in  his  mind  the  man  engaged  about  a  mine,  and  not  a  man 
who  is  accustomed  to  the  use  of  delicate  laboratory  instruments,  and 
who  has  a  well-constructed  laboratory,  with  solid  foundations  for  his 
instruments,  to  make  his  tests  in.  He  has  endeavoured  to  give  a  few 
simple  rules  that  can  be  applied  by  any  engineer  who  has  acquired  a 


PREFACE  vii 

certain  knowledge  of  electricity,  with  apparatus  that  can  be  em- 
ployed in  any  mine  in  any  part  of  the  world.  He  has  in  view 
the  possibility  that  a  mine  may  be  situated  hundreds  of  miles  from 
everywhere,  and  that  the  engineer  in  charge  of  the  apparatus  may 
have  only  himself  to  depend  upon,  may  be  obliged  to  depend  upon 
rough  and  ready  apparatus,  but  will  be  obliged  to  keep  things  going. 
The  Author  hopes  that  the  book  will  be  of  use  to  mining  en- 
gineers, mine  managers,  and  every  one  employed  about  a  mine  in  every 
capacity. 

SYDNEY  F.  WALKER. 

1,  BLOOMFIELD  CRESCENT,  BATH, 
January  18,  1907. 


CONTENTS 


CHAPTER  I 

DEFINITIONS,    UNITS,  ETC. 


What  Electricity  is        ....                  

PAGE 

1 

4 
5 
6 
8 
9 
10 
11 
14 
16 
17 
19 
9,0 

Electrical  Pressure  and  Electrical  Current 
The  Volt,  the  Ohm,  and  the  Ampere 
The  Electric  Circuit       .         . 

• 

Work  done  in  Electric  Circuits 
Heating  Effect  of  Electric  Currents 
Methods  of  producing  Electrical  Pressure 
Magnetism  and  the  Magnetic  Circuit 
Difference  between  Alternating  and  Continuoi 
Two-phase  and  Three-phase  Currents 
Electrostatic  Induction  and  the  Capacity  of  C 
Electro-magnetic  Induction  . 
Electrolysis  . 

is  Currents 
ables'         ! 

CHAPTER   II 

ELECTRIC  MINING  SIGNALS  AND   TELEPHONES 


Electric  Mining  Signals 

Sources  of  Current  for  Mining  Signals     . 

Forms  of  Galvanic  Battery     . 

Shaft  Signals 

Bells  for  Mining  Signals         ... 

Relays  for  Mining  Signals      . 

Telephones  for  Mines  , 

Telephonic  Communication  on  Engine  Roads 

Wireless  Telegraphy  for  Mines 

Shot  Firing  by  Electricity      . 

Frictional  Electrical  Firing  Apparatus    . 


22 
27 
28 
33 
35 
36 
37 
39 
40 
41 
43 


CHAPTER  III 

ELECTRIC  LIGHTING  FOR  MINES 


Arc  Lamps  .... 
The  Mechanism  of  Arc  Lamps 
Alternating-current  Arc  Lamps 


45 

47 
54 


x  CONTENTS 

PAGK 

Flame  Arc  Lamps 54 

Difference  in  the  Working  of  Continuous  and  Alternating  Current  Arc  Lamps  .  61 

Supporting  Arc  Lamps 65 

Incandescent  Lamps      .....  .  .68 

The  Gem  Carbon  Filament  Incandescent  Lamp                .                                     .  70 
Metallic  Filament  Lamps       ....                  .                                     .70 

High  C.P.  Incandescent  Lamps      ...  .  -71 

The  Nernst  Lamp .  .76 

Delivering  the  Current  to  Incandescent  and  Nernst  Lamps                                 .  79 

Portable  Electric  Lamps .80 

CHAPTER  IV 

THE  GENERATION  OF  ELECTEICITY 

The  Electricity  Generating  Station 83 

The  Possibility  of  using  Water  Power .83 

How  the  Water  Power  is  measured         .                           85 

Forms  of  Water  Engines        ....                  .....  86 

Short  Description  of  Pelton  Wheel 88 

Steam  Plant 88 

Forms  of  Water-tube  Boilers 91 

What  Combustion  is 98 

The  British  Thermal  Unit 99 

Apparatus  for  Economizing  in  Coal  and  Labour 103 

Natural,  Induced,  and  Forced  Draught 104 

Methods  of  Burning  Low-grade  Fuel 106 

The  Boiler-feed  Problem 108 

Injectors 112 

Economizers          ............  113 

Feed-water  Heaters 114 

Water  Softeners ...  116 

Grease  Extractors          ....                  ....                  .  117 

Coal  Conveyers      ............  118 

Beciprocating  Steam  Engines 119 

Working  Steam  Engines  Expansively .         .  120 

The  Government  of  Steam  Engines 126 

Difficulties  in  the  Way  of  working  Expansively       .         .         .         .   •     .         .  126 

Overcoming  Condensation  Troubles        .                   ......  128 

Superheating         .         .         . „         .  129 

Condensers .         .  134 

Forms  of  Condenser       .         .         .         .         .         .         .         .         .        .»         .  135 

Steam  Turbines 141 

The  Willans-Parsons  Steam  Turbine       .         .         .         ...         .         .  142 

The  De  Laval  Turbine .         .         .143 

The  Curtis  Turbine         . 144 

The  Westinghouse  Turbine 145 

Turbines  using  Exhaust  Steam       « 145 

Steam  Turbines  and  Condensing     .........  146 

Cooling  Towers  and  Ponds     .         .         .   :.,  ..*... 146 

Forms  of  Cooling  Towers 149 

Gas  and  Oil  Power.    Producer  and  Kindred  Gas 152 

Producer  Gas         .         .         .            ;      , 153 

The  Suction  Gas  Producer     .'•«,' 154 

The  Internal  Combustion  Engine 157 

Governing  the  Internal  Combustion  Engine   .......  159 

Cooling  the  Engine  Cylinder 160 


CONTENTS  xi 

PAGE 

Gas  Engines  for  Large  Powers 161 

Oil  Engines 163 

Governing  the  Oil  Engine 165 

The  Ignition  Problem 165 

The  Diesel  Engine 166 

A  Coal-dust  Burning  Engine  on  Similar  Lines  to  the  Diesel    ....  168 

Generators  of  Electricity 169 

The  Armature        ..........                   .  170 

The  Commutator *  174 

The  Excitation  of  Continuous-current  Machines 175 

The  Alternating  Current  Generator         ........  185 

Single,  Two,  and  Three  Phase  Alternators 189 

The  Output  and  Number  of  Poles  of  Single,  Two,  and  Three  Phase  Alternators  191 

The  Inductor  Alternator 192 

Secondary  Batteries  or  Accumulators 193 

Boosters 196 

Motor  Generators 198 

Balancers 200 

Continuous-current  Machines  with  Two  Armature  Windings  200 
Stationary  Transformers 


200 
205 
205 
206 


Transformers  for  Single,  Two,  and  Three  Phase  Currents 

Cooling  Stationary  Transformers    .  ... 

Arrangement  of  Apparatus  in  the  Generating  Station 

The  Size  of  the  Units 207 

CHAPTER  V 

DISTRIBUTION  OF  POWER  BY  ELECTRICITY 

Distribution  of  Power  by  Electricity 209 

Conductors  for  Electric  Light  and  Power  Distribution 211 

Overhead  Conductors     ...........  212 

Insulated  Conductors 215 

Bitument-covered  Cables        ..........  218 

Paper  and  Yarn-covered  Cables 219 

Fire-proof  Covering  for  Cables        .........  220 

The  Formation  of  Stranded  Conductors 221 

Concentric  Cables 221 

Three-core  Cables 224 

Sizes  of  Cables  for  Lighting  and  Power 224 

Heating  of  Cables 226 

Wires  and  Cables  for  Connecting  to  Lamps,  etc 228 

Cables  for  Coal-cutting  Machines  and  Moving  Motors      .....  229 

Fixing  Cables  in  Mines 229 

Fixing  Wires  on  Engine  Roads,  etc.        ........  234 

Methods  of  Distribution .         .235 

The  Two-wire  System 236 

The  Three-wire  System 238 

The  Use  of  the  Accumulator  as  a  Balancer      .......  241 

Distribution  by  Two  and  Three  Phase  Currents 241 

Distribution  by  Two  and  Three  Phase  Currents  at  High  Tension  and  at  Extra 

High  Tension '   .         .         .         .243 

The  Main  Switchboard 245 

The  Parallel  or  Bus  Bar  System     .        .         .        .         .         .         ...         .246 

Synchronizing  Apparatus       .         .         .         .         .         .         .         .         .         .  249 

Paralleling  Compound  Continuous-current  Machines       .        .        .         .         .  250 

The  Independent  System ^         .         .  251 

Switchboard  Gear  for  High  Tensions  and  Extra  High  Tensions        .         .         .  253 


Moving  Coil  Instruments 
Gravity  Instruments 
Hot  Wire  Instruments  . 
The  Electrostatic  Voltmeter  . 
Switches,  Fuses,  and  Circuit  Breakers 


xii  CONTENTS 

PAGE 

Sub-station  Switchboards .  254 

Measuring  Instruments  for  use  on  Main  and  Sub-station  Switchboards 

.  260 

.  261 

.  261 

.  262 

.  262 

Puses 266 

Circuit  Breakers 268 

Time  Limit  Circuit  Breakers 269 

The  Ferranti  Alternating  Current  Time  Limit  Belay  for  Circuit  Breakers          .  269 

Atkinson's  Time  Limit  Circuit  Breaker  .  270 


CHAPTER  VI 

THE  APPLICATION  OF  ELECTRICITY  TO  DRIVING 
MACHINES,  ETC.,  IN  MINES 

The  Electric  Motor 272 

Methods  of  varying  the  Speed  of  Electric  Motors 282 

The  Motor  with  Commutating  Poles .         .285 

Varying  the  Speed  of  the  Three-phase  Motor 286 

Electrically  driven  Pumps     ..........  286 

Forms  of  Pumps 287 

Ram  Pumps          ............  292 

The  Bucket  Pump 296 

Power  required  for  driving  Pumps 297 

Haulage 300 

Power  required  for  Endless  Hope  Haulage                302 

Power  required  for  Single  Drum  Haulage        .......  305 

Main  and  Tail  Haulage 306 

Power  required  for  Main  and  Tail  Haulage     ...         ....  307 

Transmitting  the  Power  from  the  Electric  Motor  to  the  Haulage  Gear     .         .  308 

Overhead  Rope  Railways .309 

Winding  by  Electricity 309 

The  Sources  of  Waste  in  Winding .310 

The  Siemens-Ilgner  Winding  Arrangement     .......  314 

Westinghouse  System  of  Electrical  Winding 317 

Winding  in  Metalliferous  Mines    ,, 322 

Coal-cutting  by  Electricity 324 

The  Process  of  Holing  or  Kirving 324 

Longwall  Coal-cutting  Machines    .         .         .         .         .         .         .         .         .  325 

Heading  Machines 327 

The  Rotary  Heading  Machine 328 

Motors  employed  with  Coal-cutting  Machines                    .         .         .         •••''.  329 

Delivering  the  Current  to  Coal-cutting  Machines     .         .         .         .         .  •       .  329 

Drilling  by  Electricity  .         .         .         .         »         .         .         .'        .         .         .  330 

Coal  Cutting  by  means  of  Drilling  Machines  .         .         .        .        .         .         .  335 

Electrically  driven  Fans         .         .         .         .         ...         .         .         .  336 

Description  of  Different  Forms  of  Fans 336 

The  Sirocco  Fan    .     ^    .         .         • j       ,.        V        ...         .         .         .  338 

Power  required  for  driving  Fans     .         '-,         .         .      *  .         .         .         .         .  340 

Driving  Air  Compressors        .               ; 341 

Power  required  for  driving  Air  Compressors  Underground         ....  345 

Electric  Locomotives 346 

The  Electric  Driving  of  other  Machinery  about  the  Mines       ....  347 
Estimating  the  Power  given  out  by  Steam  or  Compressed  Air  Engines  that  are 

to  be  displaced  by  Electric  Motors 348 


CONTENTS  xiii 

CHAPTER  VII 

FAULTS  IN  ELECTRICAL  APPARATUS 

PAGE 

Faults  in  Electrical  Apparatus 360 

Rules  for  Testing 351 

Faults  in  Mine  Signals 352 

Faults  in  Telephones 355 

Faults  in  Dynamo  Machines  ..........  357 

Faults  in  Continuous-current  Armatures 357 

Testing  for  Disconnection  in  Continuous-current  Armatures    ....  359 

Turning  up  Commutators 360 

Testing  for  Disconnection  in  an  Alternating-current  Armature          .                   .  361 

Conductivity  and  Insulation  Tests  in  Dynamos        ......  362 

Testing  for  Insulation 364 

Trouble  with  the  Brush  Holders 365 

Faults  in  Cables 366 

Finding  a  Short  Circuit  between  Cables           .......  368 

Testing  Cables  for  Disconnection    .                           369 

Tests  by  Electrostatic  Capacity       .         .                  370 

Faults  in  Switch  Gear   .         .  371 


INDEX 373 


LIST   OF   ILLUSTRATIONS 


FIG.  PAGE 

1.  Diagram  of  Series  Connections 7 

2.  Ditto,  Parallel  Connections 7 

3.  Ditto,  Parallel  Series  Connections 8 

4.  Ditto,  Two-wire  Engine  Road  Signal,  with  One  Bell         ....  22 

5.  Ditto,  ditto,  Two  Bells 23 

6.  Ditto,  Three-wire  Engine  Road  Signal,  with  Two  Bells    ....  24 

7.  Ditto,  ditto,  with  several  Bells  in  Series 25 

8.  Ditto,  ditto,  in  Parallel 25 

9.  Reel  Insulators  for  Engine  Road  Signals 30 

10.  Vulcanized  Rubber  Insulators  for  Engine  Road  Signals    ....  31 

11.  Shackle  Insulators  for  Engine  Road  Signals 31 

12.  Diagram  of  Shaft  Signals 32 

13.  Ringing  Key  for  Shaft  Signal 33 

14.  Ditto,  ditto,  worked  by  Long  Lever 34 

15.  Sectional  Drawing  of  Ringing  Key  for  Shaft  Signal 34 

16.  Gas  Proof  Trembler  Bell,  for  Mining  Signals 35 

17.  Ditto,  Single  Stroke,  for  Mining  Signals 35 

18.  Diagram  of  Relay  for  Engine  Road  Signals 36 

19.  Iron-cased  Relay  for  Mining  Signals        .......  37 

20.  Special  Telephone  Set  for  Mining  Work 40 

21.  22.  Sections  of  Johnson  and  Phillips'   Single-carbon,  Brake  Mechanism 

Arc  Lamp 47 

23.  Mechanism  of  Union  Co.'s  Open  Arc  Lamp      ......  49 

24.  Sectional  Drawing  of  "  Jandus  "  Enclosed  Arc  Lamp        ....  50 

25.  Ditto,  Johnson  and  Phillips'  "  Ark  "  Enclosed  Arc  Lamp          ...  50 

26.  Diagram  of  the  B.  T.  H.  Co.'s  Enclosed  Arc  Lamp 51 

27.  "Angold"  Enclosed  Arc  Lamp,  showing  Mechanism  and  Resistance          .  51 

28.  Mechanism  of  Union  Co.'s  Enclosed  Arc  Lamp 52 

29.  Ditto,  "  Angold  "  Double  Carbon,  Open  Type  Arc  Lamp   ....  53 

30.  Ditto,  ditto,  ditto,  for  Rectified  Currents 55 

31.  Sectional  Drawings  of  "  Excello  "  Flame  Arc  Lamp          ....  56 

32.  Curves  showing  Light  given  by  "  Excello  "  and  ordinary  Arc  Lamps          .  57 

33.  Sectional  Drawings  of  "  Juno  "  Flame  Arc  Lamp 58 

34.  Diagram  of  Connections  for  Two  and  Four  Arc  Lamps  to  Service  Mains    .  59 

35.  Diagram  of  Arrangement  for  Working  Lamps  from  a  Motor  Generator       .  60 

36.  Ditto,  ditto,  for  Arc  Lamps  in  Parallel  across  a  65  or  100  volt  service         .  61 

37.  Automatic  Cutout  for  Continuous-current  Arc  Lamp         ....  62 

38.  Diagram  of  Alternate-current  Arc  Lamps,  in  Series,  with  Balancing  Coils  63 

39.  Diagram  of  Connections  for  Two  Arcs  with  Compensator  on  an  Alternate- 

current  Service    .         .         .         .         .         .         •      •  •         .        ..64 

40.  "  Economy "  Coil,  or  Compensator .         .         .         .         .        .         .         .64 

41.  Lattice-work  Iron  Lamp  Post  for  Arc  Lamp 66 

42.  43.  Sectional    Drawings   of   Schaeffer's    Suspension    Apparatus  for  Arc 

Lamps 67 

44.  Parts  of  Incandescent  Lamp  Holder 73 

xv 


XVI 


LIST   OF  ILLUSTRATIONS 


PIG.  PAGE 

45.  Forms  of  Incandescent  Lamp  Holders 74 

46,  47,  48.  "  Glower  "  and  Compensating  Resistance  of  Nernst  Lamp      .         .  76 
49,  50.  Complete  Nernst  Lamps,  with  Shades,  etc 77 

51.  Diagram  of  Connections  of  Nernst  Lamp          ......  78 

52.  Curves  showing  Light  given  by  Nernst  Lamp 79 

53.  Sectional  Drawings  of  Davy  Paxman's  "  Economic  "  Boiler      ...  90 

54.  Ditto,  "  Stirling  "  Boiler 93 

55.  Ditto,  "  Climax  "  Boiler         - 97 

56.  Sectional   Drawing    showing   Arrangement    for    Burning    Coal-dust    in 

Lancashire  Boiler         ..........  107 

57.  Ditto,  Worthington  Plunger  and  Ring  Pump 110 

58.  Ditto,  Showing  Connection  of  Injector  to  Lancashire  Boiler      .         .         .  112 

59.  Ditto,  Ditto,  Water-tube  Boiler       ....<,...  113 

60.  Ditto,  Ditto,  Feed  Water  Heater 114 

61.  62.  Royle's  Feed  Water  Heater .115 

63.  One  Form  of  Wells'  Oil  Filter 117 

64.  Sectional  Drawings  of  Bellis'  Compound  Engine 123 

65.  Ditto,  Ditto,  Triple-expansion  Engine 125 

66.  Tinker's  Superheater,  fixed  in  a  Lancashire  Boiler  .....  130 

67.  Longitudinal  Section  of  Davy  &  Paxman's  Superheater  ....  131 

68.  Transverse  Section  and  Diagram  of  Ditto 132 

69.  Curves  showing  the  Advantages  of  Superheating       .....  133 

70.  Electrically-driven  Edwards'  Triple  Air  Pump 134 

71.  Sectional  Drawing  of  Edwards'  Air  Pump 135 

72.  Ledward's  Evaporative  Condenser  ........  136 

73.  Sectional  Drawing  of  Worthington  Jet  Condenser    .         .         .     ,    .  137 

74.  Ditto,  Ledward's  Ejector  Condenser         .         .         .                  .         .         .  138 

75.  Sectional  Diagram  of  Worthington  Central  Condenser      .         .         .  ,       .  139 

76.  Ditto,  ditto,  Central  Condensing  Station 140 

77.  Ditto,  ditto,  Cooling  Tower 150 

78.  Ditto,  Campbell's  Suction  Gas  Producer 156 

79.  Ditto,  Oechelhausen  Two  Cycle  Gas  Engine 162 

80.  Sectional  Drawings  of  Diesel  Oil  Engine          ......  167 

81.  Commutator  of  Continuous  Current  Generator 175 

82.  Diagram  of    Connections  of    Continuous    Current   Separately  Excited 

Generator    .         .         .  ,        ,         .         .         .         ..        .         .176 

Ditto,  Series-wound  Generator 

Ditto,  Shunt-wound  Generator 

Ditto,  ditto,  with  Adjustable  Field  Current 

Ditto,  Compound-wound  Generator 

Messrs.  Santoni's  Brush  Holder 


83. 

84. 

85. 

86. 

87. 

88,  89.  Diagram  of  Connections  of  Two  and  Three 

90. 

91. 

92, 


Phase  Armatures 


High-tension  Insulators 
A  Special  Form  of  High-tension  Insulator 

93.  Diagrams  of  Mavor  and  Coulson's  Concentric  Cables  and  Methods  of 
Jointing  them 

94.  Diagram  showing  Method  of  connecting  a  Branch  to  Mavor  and  Coulson's 

Concentric  Cables 

95.  Messrs.  Callender's  Single  Cable  Cleat  for  Mine  Shafts 

96.  Ditto,  Two  Cable  Cleat  for  Ditto     .         .         .         ,. 

97.  Ditto,  Single  Cable  Cleat,  supported  by  Chains 

98.  Messrs.  Glover's  Method  of  jointing  Cables  in  a  Shaft 

99.  Diagram  of  Two-wire  System  of  Distribution  . 

100.  Ditto,  Three-wire  System,  with  Two  Generators 

101,  102.  Diagrams  of  Three-wire  System,  with  one  Generator 

103.  Ditto,  Connections  for  Three-phase  High  Tension  Distribution 

104.  Ditto,  ditto,  Three-phase  Extra  High  Tension  Distribution 

105.  Ditto,  ditto,  Two  Shunt-wound  Generators  to  Bus  Bars 

106.  Ditto,  ditto,  Two  Separately  Excited  Machines  to  Bus  Bars 

107.  Ditto,  ditto,  Three-phase  Generators  to  Bus  Bars 248 


177 

178 
179 
180 
184 
190 
213 
214 

222 

223 
230 
231 
232 
233 
237 
239 
240 
243 
244 
247 
247 


LIST   OF  ILLUSTRATIONS  xvii 

PIG.  PAGE 

108.  Diagram  of  Connections  of  Two  Compound  Machines  to  Bus  Bars    .         .     250 

109.  Ditto,  ditto,  a  Sub-station  at  a. Colliery,  to  take  Current  from  a  Power 

Service 255 

110.  Sectional  Diagram  of  the  Back  of  the  Switchboard  shown  in  previous 
Figure 256 

111.  112.  Diagrams  of  Berry,  Skinner  and  Co.'s  Distributing  Boards         .         .     257 

113.  Sectional  Diagram  of  Mavor  and  Coulson's  Distributing  Fuse  Box  .         .     258 

114.  Double-pole  Switch  in  Iron  Case 264 

115.  Iron-cased  Fuses  for  Mining  Work 268 

116.  Diagram  of  Connections  for  Starting  a  Series-wound  Motor      .         .         .273 

117.  Ditto,  ditto,  ditto,  a  Shunt-wound  Motor 273 

118.  Ditto,  ditto,  Johnson  and  Phillips'  Motor  Starting  Box    .         .         .         .274 

119.  120.  Ditto,  ditto,  the  B.  T.  H.  Co.'s  Form  A  Starting  Box        .         .         .276 
121, 122.  Ditto,  ditto,  ditto,  Form  B 277 

123.  Sectional  Drawing  of  Messrs.   Beyrolle's  Packing   Ring  for    Gas-tight 

Motor  Starters 278 

124.  Diagram  of  Connections  for  Starting  Three-phase  Motors          .         .         .280 

125.  Ditto,  Westinghouse  Three-phase  Motor  Starting  Boxes  ....     281 

126.  Ditto,  Connections   for    regulating    Speed   of    Series-wound    Motor    by 

Variation  of  Pressure  ..........     282 

127.  Ditto,  ditto,  for  varying  Speed  of  Series-wound  Motor  by  Varying  Field 

Current 284 

128.  Ditto,  ditto,  for  Varying  Speed  of  Shunt-wound  Motor    .         .         .         .285 

129.  Sectional  Diagram  of  Worthington  Multistage  Centrifugal  Pump      .         .  289 

130.  Efficiency  Curve  of  Centrifugal  Pump,  with  Constant  Quantity         .         .  290 

131.  Ditto,  ditto,  with  Constant  Speed 291 

132.  Sectional  Diagram  of  Eiedler  Differential  Pump 294 

133.  134.  Gutermuth  Valves 295 

135.  Sectional  Drawing  of  David  Bridge's  Friction  Clutch       .         .         .         .300 

136,  137.  Friction  Clutches,  and  Arrangement  for  Endless  Eope  Driving  .         .     301 

138.  Diagram  of  Connections  of  Three-phase  Winding  Plant  at  Preussen  II. 

Colliery 311 

139.  Side  Elevation  of  Electric  Winding  Plant  at  Zollern  II.  Colliery       .         .  312 

140.  Plan  of  Winding  Gear  at  Zollern  II.  Colliery 313 

141.  Diagram  of  Siemens-Ilgner  System  of  Electric  Winding  ....  316 

142.  Ditto,  Westinghouse  System  of   Winding  from   a  Three-phase  Supply 

Service        . 318 

143.  Diagram  of  Westinghouse  Converter-Equalizer  System     ....  320 

144.  Diagram  of  Control  Gear  of  Westinghouse  Electric  Winding  Plant  .         .  321 

145.  Plan  of  Whiting  System  of  Winding  from  Deep  Metalliferous  Mines         .  323 

146.  Sectional  Drawing  showing  Operation  of  Disc  Coal-cutting  Machine          .  326 

147.  Vertical  Sectional  Drawing  of  Machine  shown  in  previous  Figure     .         .  327 

148.  149.  Sectional  Drawings  of  Siemens'  Rotary  Electric  Drill        .         .         .331 

150.  Ditto,  ditto,  of  Siemens'  Percussion  Drill        .  ...     332 

151,  152.  Sectional  Drawings  of  Motor  Cases  for  Siemens'  Drills,  where  the 

Motors  are  separate      ..........  333 

153,  154.  Ditto,  Denver  Electric  Air  Drill 334 

155.  Ditto,  Marvin  Electric  Drill 334 

156.  Diagram  showing  Relative  Efficiencies  of  Compressed  Air  when  Com- 

pressor is  driven  at  Bank  and  in-Bye    .         .         .         .         .         .  344 

157.  Lineman's  Detector  Galvanometer 353 

158.  Diagram  of  Connections  for  testing  a  Telephone  Set         ....     356 

159.  Ditto,  ditto,  for  a  Break  in  an  Armature          .         ,         .         .         .         .359 

160.  Ditto,  ditto,  for  a  Break  in  a  Field  Coil .        »        .         .         •         •         .363 

161.  Ditto,  ditto,  Insulation  Resistance  of  an  Armature  .....     364 

162.  Ditto,  ditto,  ditto,  Armoured  Cable          .         .         .         .         .         •         .366 

163.  Ditto,  ditto,  ditto,  Unarmoured  Cable 367 


LIST    OF    PLATES 


PLATE                                                                                                                                                                                          TO  PACE  PACK 

Electric  Winding  Plant  at  Lens  Colliery  ....         Frontispiece 

IA.  Babcock  and  Wilcox  Water-tube  Boiler 88 

IB.  Climax  Water-tube  Boiler 88 

2A,  B,  and  c.  Hodgkinson's  Mechanical  Stoker 104 

2D.  Carter's  Economizer ....  104 

SA.  Green's  Economizer 112 

SB.  Eoyle's  Water  Softener 112 

4A.  Bellis'  Triple  Expansion  Engine,  with  Generator 128 

4B.  Willans-Par sons'  Steam  Turbine 128 

5.  Oechelhausen  Gas  Engine,  driving  Dynamo       ....                   .  144 

GA.  Korting  Gas  Engine       ..........  152 

GB.  Mather  and  Platt's  Multipolar  Dynamo 152 

TA.  Field  Magnet  Ring,  with  Magnets  and  Brush  Holders      ....  160 

7B.  A  Motor  Generator 160 

SA,  B,  and  c.  Dick-Kerr  &  Co.'s  Turbo  Alternator 168 

9A.  A  Sub-station  of  Rotary  Converters         .......  184 

9B.  A  Water-power  driven  Generating  Station 184 

10.  An  Oechelhausen  Gas-engine  driven  Generating  Station  ....  192 

11.  A  Suction  Gas  Producer  Generating  Station 208 

12A  and  B.  Reyrolle's  Switchboard 216 

13A.  A  Siemens  Switchboard 224 

13s.  A  Westinghouse  Sub-station  Board,  for  underground        ....  224 

13c.  Ferranti  Triple-pole  Circuit  Breaker 224 

14A  and  B.  Ferranti  Oil  Enclosed,  10,000  Volts  for  Switches    .         .         .         .232 

14c  and  D.  Ferranti  Enclosed  Three-phase  Mining  Switches     .         .                  .  232 

15A  and  B.  Siemens'  Motor  Generator  Switchboard 240 

15c.  Dorman  and  Smith's  Gas  and  Fool  Proof  Switch 240 

16A  and  B.  Westinghouse  Distributing  Boxes  for  Mines 248 

16c  and  D.  Ferranti  Circuit  Breakers 248 

17A  and  B.  Current  and  Pressure  Transformers  for  Switchboard  Instruments    .  256 

17c.  Ferranti  Gas-proof  Switch 256 

17D.  Reyrolle's  Oil  Enclosed  Three-phase  Switch 256 

18A.  Handle  Fuses 264 

18s,  c,  and  D.  Ferranti  Circuit  Breakers 264 

19A.  Parts  of  Continuous-current  Motor           .......  272 

19s,  c,  and  D.  Three-phase  Motor,  and  parts  of  ditto 272 

20A  and  B.  Reyrolle's  Motor  Starting  Switch 280 

20c.  The    International    Electrical    Co.    Reversing    Apparatus,    for    Electric 

Winding,  as  used  at  Waihi  Junction  Mine,  New  Zealand     .         .         .  280 

2lA.  Dorman  and  Smith's  Liquid  Starting  Resistance 288 

2lB.  Motor  Starting  Panel .288 

21c.  Reyrolle's  Enclosed  Mining  Switch          .         ...         .         .         .         .288 

22 A  and  B.  Siemens'  Motor  Switchboard .  296 

22c.  Wound  Rotor  for  Two-phase  Motor ,         .296 

22o.  Diamond  Coal  Cutting  Co.'s  Electric  Rotary  Drill 296 

xix 


XX 


LIST  OF  PLATES 


PLATK  TO  FACE  PAGE 

23A.  High  Lift  Centrifugal  Pump,  driven  by  Electric  Motor  .  .  .  .304 
23B.  Mather  and  Platt's  Variable  Speed  Three-throw  Bam  Pump  Electrically 

Driven 304 

24A.  Worthington  Centrifugal  Pump,  Electrically  Driven,  arranged  for  Sinking  312 

24B.  Electrically  Driven  Three-throw  Sinking  Pump 312 

25A.  Electrically  Driven  Centrifugal  Pump,  suspended  over  Shaft,  ready  for 

Sinking ....  320 

25B.  Dick-Kerr  &  Co.'s  Electric  Loco,  with  Bow  Trolley          .         .         .         .320 

26A.  Electrically  Driven  Three-throw  Dip  Pump    .  ....     328 

26B.  Electric  Mine  Loco,  coming  out  of  a  level        .  ....     328 

27A.  Electrically  Driven  Endless  Rope  Haulage  Plant  ....     336 

27fi.  Electrically  Driven  Dip  Haulage  Plant  .  ....     336 

27c.  Electrically  Driven  Main  and  Tail  Haulage  Plant  .         .         .         .336 

28A.  Electrically  Driven  Winding  Plant          .  ....     344 

28s.  Electric  Winding  Motor  and  Brake  used  at  Waihi  .         .         .         .344 

29A.  Clarke-Steavenson's  Disc  Coal-cutting  Machine,  with  Worm  Gearing        .     352 
29B.  Diamond  Coal  Cutting  Co.'s  Disc  Machine       .  ....     352 

29c.  Pickquick  Bar  Coal-cutting  Machine       .  ....     352 

29D.  Electrically  Driven  Chain  Coal-cutting  Machine  ....     352 

30A.  Heenan  and  Froude's  Fan,  without  case.         .  ....     360 

30B.  Heenan  and  Froude's  Fan,  in  -case,  electrically  driven       ....     360 

30c.  Fan  House,  Electrically  Driven  Fan  for  Three-phase  Service    .         .         .     360 
3lA  and  B.  Eeavell's  Electrically  Driven  Air  Compressors         ....     368 

31c  and  D.  Blackett's  Underground  Coal  Conveyer,  Electrically  Driven  .  .  368 


- 


ELECTRICITY    IN    MINING 

CHAPTER  I 

DEFINITIONS,  UNITS,  ETC. 

What  Electricity  is 

AT  the  present  time  there  are  two  theories  of  electricity,  one  held  by 
the  advanced  section  of  the  professors  led  by  Professor  J.  J.  Thompson, 
of  Cambridge,  and  known  as  the  corpuscular  or  electrotonic  theory  ; 
and  the  other  held  by  those  practical  engineers  who  have  thought 
about  the  matter,  and,  the  author  believes,  by  a  section  of  the 
professors  led  by  Sir  Arthur  Kucker,  the  Principal  of  the  London 
University,  known  as  the  wave  or  molecular  theory.  In  order  to 
understand  the  corpuscular  theory,  it  is  necessary  to  briefly  explain 
the  atomic  theory,  which  those  mining  engineers  who  have  studied 
chemistry  will  already  have  met  with.  In  the  atomic  theory  there 
are  a  certain  limited  number  of  substances  which  are  indivisible 
into  other  substances,  so  far  as  any  means  at  the  disposal  of  the 
laboratories  is  able  to  accomplish  at  present,  these  substances  being 
known  as  the  chemical  elements.  The  elements  combine  with  each 
other  to  form  compounds,  the  compounds  possessing  different 
properties  from  those  of  the  elements  out  of  which  they  are  formed, 
and  having  a  different  appearance.  The  elements  always  combine  in 
certain  fixed  proportions,  these  being  the  atoms.  One  or  more 
atoms  of  any  element  may  combine  with  one  or  more  atoms  of 
other  elements  to  form  one  or  more  molecules  of  a  compound.  The 
atom  was  supposed  to  be  the  smallest  body  that  could  exist,  and  it 
could  not  exist  alone.  Two  or  more  atoms  of  an  element  might 
exist  together  as  a  molecule  of  that  element.  The  molecules  are 
supposed  to  be  all  the  same  size,  but  the  atoms  are  of  different  weights, 
and  the  atomic  weight  is  that  quantity  by  weight  of  each  element 
that  enters  into  combination  with  one  or  more  atoms  of  other  elements. 
In  the  corpuscular  theory,  the  atom  is  again  supposed  to  be  divided 
into  an  enormous  number  of  smaller  bodies  known  as  corpuscles  or 

i  B 


2  ELECTRICITY  IN   MINING 

electrons.  There  are  an  equal  number  of  positively  electrified  and 
negatively  electrified  corpuscles  in  every  atom  in  its  normal  condition. 
The  negatively  electrified  corpuscles  or  electrons  are  supposed  to 
completely  surround  and  enclose  the  positively  electrified  electrons. 
The  negative  electrons  are  thrown  off  from  the  surface  of  the  atom 
with  great  velocity,  and  when  an  atom  has  lost  one  or  more  negative 
electrons,  it  becomes  positively  electrified,  and  exercises  an  attraction 
for  any  body  which  is  short  of  positive  electrons.  On  this  theory 
an  electric  current  is  supposed  to  be  a  procession  of  electrons  in  the 
space  surrounding  the  conductor  through  which  a  current  is  supposed 
to  pass.  The  corpuscular  theory  has  been  led  up  to  from  the 
laboratory,  and  mainly  from  the  researches  that  have  been  made  on 
the  Crookes'  tubes  used  with  X-ray  apparatus.  The  cathode,  the 
negative  electrode  in  the  Crookes'  tube,  has  been  found  to  give  off  a 
large  number  of  negative  corpuscles  at  a  very  high  velocity. 

The  wave,  or  molecular,  or  mechanical  theory  of  electricity  has 
been  led  up  to  from  the  principles  that  have  been  gradually  evolved — 
since  the  wave  theory  of  light  enunciated  by  Dr.  Thomas  Young  took 
the  place  of  the  corpuscular  theory  of  light  previously  held  by  Sir 
Isaac  Newton  and  others.  Sir  Isaac  Newton  believed  that  light  came 
to  us  from  the  sun,  very  much  in  the  same  way  as  the  enunciators  of 
the  corpuscular  theory  of  electricity  believe  that  electric  currents  are 
formed.  The  wave  theory  took  its  place,  however,  among  scientific 
men,  even  in  Newton's  lifetime,  for  reasons  there  is  not  space  to 
detail  here.  Dr.  Young  propounded  the  theory  that  light  comes  to 
us  in  waves,  somewhat  similar,  but  different  in  form,  to  the  waves 
that  had  already  been  shown  to  be  the  method  of  the  propagation  of 
sound,  and  that  we  are  familiar  with  in  water. 

The  Ether. — In  order  that  light  should  come  to  us  in  waves,  it 
was  necessary  that  there  should  be  a  medium  for  the  passage  of  the 
waves,  hence  the  invention  of  the  ether  by  the  scientific  men  of  those 
days.  It  is  now  recognized  that  our  sun,  his  planets,  and  the  whole 
of  the  heavenly  bodies  float  in  an  elastic  fluid,  whose  properties  are 
not  yet  fully  understood,  and  which  goes  by  the  name  of  the  Ether. 
White  light,  as  we  know,  is  made  up  of  a  number  of  coloured  lights, 
the  beautiful  colours  of  the  rainbow  or  spectrum,  red  being  at  one  end 
of  the  spectrum  and  violet  at  the  other.  It  is  also  now  known  that 
the  different  colours  are  due  to  differences  in  the  lengths  of  the  waves 
by  which  the  different  colours  are  transmitted.  The  length  of  the 
wave  in  the  red  is  approximately  3-3^00  °^  an  *nck>  wn^e  tnat  °f  tne 
wave  in  the  violet  is  about  half  that  length.  But  it  is  also  well 
known  now  that  the  different  colours  and  the  different  wave-lengths 
correspond  to  different  properties ;  thus,  red  rays  have  greater  heat- 
ing property,  while  violet  rays  have  greater  actinic  or  chemical 
properties,  and  the  yellow  rays,  which  are  near  the  middle  of  the 


DEFINITIONS,   UNITS,   ETC.  3 

spectrum,  have  the  greatest  lighting  value.  Further,  there  are  dark 
rays  beyond  the  red,  known  as  the  infra  red,  the  wave-length  of 
which  is  longer  than  that  of  the  red ;  and  there  are  also  dark  rays 
beyond  the  violet,  known  as  the  ultra  violet,  which  have  greater 
chemical  power  than  the  violet  rays,  and  whose  wave-length  is 
shorter  than  the  violet.  It  will  be  evident  that  there  is  a  long  range 
of  wave-lengths  below  the  infra  red  and  above  the  ultra  violet  that 
have  not  yet  been  located,  and  whose  properties  are  not  yet  known, 
but  that  are  being  gradually  investigated.  Further,  when  an  electric 
current  is  caused  to  heat  a  conductor,  if  the  current  is  sufficiently 
powerful,  and  is  allowed  to  pass  for  a  sufficient  length  of  time,  first, 
invisible  heat  rays  are  produced  ;  next,  visible  red ;  then  yellow ;  and 
finally  white  rays.  Hence  it  was  but  a  natural  sequence  of  the 
acceptance  of  the  wave  theory  of  light,  that  the  wave  or  mechanical 
theory  of  heat  should  follow,  and  then  that  the  wave  or  mechanical 
theory  of  electricity  should  follow  the  wave  theory  of  heat.  It  has 
also  been  proved,  by  the  researches  of  Hertz  and  others,  that  electric 
waves  are  created,  and  they  are  used  in  wireless  apparatus,  of  which 
Marconi's  is  the  best  known. 

The  View  for  the  Practical  Engineer.— The  above  has  been 
detailed,  though  it  is  not  absolutely  necessary,  because  the  author 
believes  that  the  engineer  of  the  present  day  takes  a  great  and 
increasing  interest  in  every  development  of  every  science  that  he  has 
to  handle,  and  most  engineers  have  to  handle  several  sciences.  The 
practical  engineer  has  to  remember,  however,  that  by  the  law  of 
the  conservation  of  energy,  electricity,  no  matter  what  may  be  its 
form,  is  only  obtained  by  transformation  from  some  other  form 
of  energy. 

The  Law  of  the  Conservation  of  Energy. — In  these  days  when 
researches  upon  radium  and  helium  are  supposed  to  have  upset  some 
of  the  laws  that  were  supposed  to  govern  the  universe,  it  is  wise  for 
the  practical  engineer  to  remember  what  the  law  of  the  conservation 
of  energy  is,  and  to  keep  close  to  it.  This  law  states  that  the 
quantity  of  matter  and  the  quantity  of  energy  in  the  universe  are 
fixed  and  unchangeable,  and  that  neither  matter  nor  energy  can  be 
lost  or  created.  Hence,  when  we  talk  of  creating  or  generating,  say, 
electricity,  we  really  mean — we  can  only  mean — that  we  have  trans- 
formed energy  from  some  other  form  into  electricity,  and  that  in 
whatever  form  electricity  is  generated,  to  use  the  convenient  expres- 
sion, work  must  have  been  done,  or,  as  we  say,  some  other  form  of 
energy  must  have  been  expended. 


ELECTRICITY   IN   MINING 


Electrical  Pressure  and  Electrical  Current 

The  term  pressure  is  used  by  electrical  engineers  in  the  same  way 
as  it  is  employed  by  mechanical  and  civil  engineers.  It  implies  the 
presence  of  a  force  that  will  give  rise  to  an  electrical  current  when 
the  other  electrical  conditions  are  favourable.  Put  in  another  way, 
an  electrical  current  will  pass  between  any  two  points  where  there  is 
a  sufficient  electrical  pressure  between  these  two  points  to  overcome 
the  resistance  opposed  to  the  passage  of  the  current.  Further,  the 
electrical  current  that  will  pass  between  the  two  points  will  be 
directly  proportional  to  the  pressure  existing  between  them,  and 
inversely  proportional  to  the  resistance  opposed  to  the  passage  of 
electricity. 

Ohm's  Law.  —  The  above  law,  which  governs  so  much  of  the 
action  of  electric  currents,  is  known  by  the  name  of  the  celebrated 
German  professor  who  discovered  it.  It  must  be  understood,  how- 
ever, that  the  pressure  employed  in  applying  Ohm's  law  is  the  net 
pressure  present  between  the  two  points.  It  happens  in  many  cases, 
as  in  primary  and  secondary  galvanic  batteries,  and  in  apparatus 
where  magneto-electric  induction  takes  place,  that  opposing  pressures 
exist  between  the  two  points  in  question.  When  this  is  the  case,  the 
pressure  to  be  employed  is  the  algebraical  sum  of  all  the  pressures 
existing  between  the  two  points.  A  good  illustration  of  this  is 
where  accumulators  are  being  charged.  The  charging  dynamo 
furnishes  a  current  of  a  certain  pressure;  the  accumulator,  being 
charged,  furnishes  an  opposing  pressure  nearly  equal  to  it  ;  and  the 
resultant  current,  passing  through  the  accumulator,  the  cables,  etc.,  is 
found  by  applying  Ohm's  law,  but  using  the  difference  between  the 
charging  and  opposing  pressures  as  the  pressure  for  calculation. 

Ohm's  law  is  written  — 


also  — 

E  =  CK, 
and  — 

E 


The  second  and  third  formulae  will  be  recognized  as  merely  algebraical 
transpositions  of  the  first.  Also,  when  E  is  in  volts,  and  E  is  in 
ohms,  C  is  in  amperes. 


DEFINITIONS,   UNITS,   ETC. 


The  Volt,  the  Ohm,  and  the  Ampere 

The  Volt  is  the  unit  of  electrical  pressure.  It  is  used  by 
electrical  engineers  in  very  much  the  same  way  as  the  pound  is 
used  by  mechanical  engineers.  It  is  a  definite  multiple  of  the 
standard  electrostatic  unit,  the  force  exerted  by  a  body  of  unit 
volume,  charged  to  unit  electrical  potential  upon  another  body 
similarly  charged  at  unit  distance.  The  practical  engineer  need  not 
trouble  himself  very  seriously  about  the  standard  units.  The  volt 
is  about  equal  to  two-thirds  the  pressure  that  the  Leclanche  cell,  so 
much  employed  in  electric  signals,  telephones,  etc.,  should  have  when 
giving  no  current.  It  is  also  about  equal  to  two-thirds  the  pressure 
of  the  standard  Clark  cell  that  is  employed  in  laboratory  work,  and  in 
delicate  tests  of  electrical  apparatus. 

The  Ohm  is  the  unit  of  electrical  resistance.  All  bodies  resist 
the  passage  of  electricity  through  them,  the  metals  less  than  other 
bodies,  and  silver  and  copper  the  least  of  the  metals,  while  substances 
such  as  dry  cotton,  dry  silk,  indiarubber,  bitumen,  porcelain,  and 
others,  offer  a  very  high  resistance  indeed  to  the  passage  of  electricity. 
Substances  are  roughly  divided  into  conductors  and  insulators,  the 
conductors  being  mainly  the  metals,  and  the  insulators  the  substances 
mentioned  above,  and  some  others.  The  standard  unit  of  resistance 
is  a  column  of  mercury  of  certain  dimensions  kept  at  Paris.  For 
practical  purposes  it  is  perhaps  more  important  to  know  that  1  mile 
of  No.  4  copper  wire,  J  mile  of  No.  8,  and  other  lengths  of  other 
wires  have  approximately  a  resistance  of  one  ohm. 

Specific  Resistance. — What  is  known  as  the  specific  resistance 
of  any  substance  is  the  ratio  which  the  resistance  offered  by  a  cubic 
centimeter,  or  cubic  inch  of  the  substance  between  two  opposing 
faces,  bears  to  that  offered  by  a  cubic  centimeter  or  cubic  inch  of 
silver.  The  specific  resistance  of  iron  and  steel  are  from  six  to  seven 
times  that  of  silver  and  copper.  The  specific  resistance  of  the 
insulators  are  some  of  them  many  million  times  that  of  silver  and 
copper.  The  resistance  of  every  body  of  any  substance  is  propor- 
tional, directly  to  the  length  of  the  body  in  the  direction  in 
which  the  current  will  pass,  and  inversely  to  its  sectional  area  in  the 
same  direction.  Thus  the  resistance  of  a  copper  conductor,  intended 
to  carry  a  lighting  or  power  current,  varies  directly  as  its  length,  and 
inversely  as  its  size,  while  the  resistance  of  the  indiarubber  or  other 
insulating  envelope  varies  directly  as  the  thickness  of  the  envelope, 
and  inversely  as  the  length  of  the  conductor  it  envelops.  The 
resistance  of  all  substances  also  varies  with  the  temperature.  The 
resistance  of  all  the  metals  increases -as  the  temperature  rises,  by  a 
definite  fraction,  that  of  carbon  and  some  other  substances  decrease. 


6  ELECTRICITY   IN   MINING 

The  Ampere  is  the  unit  of  current,  and  it  is  that  which  flows 
through  any  conductor  under  a  pressure  of  one  volt  when  opposed  by 
a  resistance  of  one  ohm,  or  any  multiples  of  these  numbers.  Thus 
one  ampere  will  pass  under  a  pressure  of  100  volts,  opposed  by  a 
resistance  of  100  ohms. 


The  Electric  Circuit 

The  electric  circuit  is  the  path  of  the  current  that  is  intended  to 
perform  useful  work,  and  it  must  include  the  generator,  the  apparatus 
that  is  to  be  worked,  the  apparatus  that  controls  the  passage  of  the 
current  through  the  circuit,  such  as  the  switch  or  the  push,  and  the 
wires  or  cables  connecting  all  together.  There  may  be,  and  usually  are, 
several  circuits  having  the  generator  common  to  all  of  them,  but  the 
same  remark  applies  to  each  of  the  branch  circuits,  as  they  are 
termed.  The  terms  open  circuit  and  closed  circuit  are  expressions 
meaning  that,  in  the  first  place,  the  circuit  is  not  complete,  and  there- 
fore the  apparatus  that  is  to  be  worked  by  the  passage  of  a  current 
through  it  does  not  work,  and  in  the  other  case  that  it  is  complete,  and 
that  the  apparatus  should  work.  The  term  breaking  circuit  is  also 
employed,  and  means  opening  the  circuit,  destroying  its  continuity, 
as  when  a  switch  is  thrown  back,  or  when  a  wire  or  cable  is  parted. 
Ohm's  law  applies  to  the  working  of  all  electric  circuits,  whether 
single  or  composed  of  several  branches.  The  current  which  passes 
in  any  circuit  or  in  any  branch  circuit  is  determined  by  Ohm's  law, 
but  in  the  whole  circuit  the  whole  of  the  resistance,  including  that  of 
the  generator,  must  be  taken  into  account  in  calculating  the  current 
that  will  pass,  and  in  any  branch  circuit  the  pressure  that  exists 
between  the  ends  of  each  branch  and  the  resistance  of  the  branch. 
For  a  main  circuit  the  formula  usually  stands  thus — 


where  C  is  the  current  passing  in  the  whole  circuit,  Ea  is  the 
resistance  of  the  generator,  K&  that  of  the  apparatus  to  be  worked,  and 
K«  that  of  the  cables  or  wires  connecting  them  together. 

It  would,  perhaps,  be  more  correct  to  say  that  the  above  describes 
the  useful  circuit.  A  circuit  exists  wherever  a  path  for  the  current 
exists.  There  is  a  leakage  circuit,  for  instance,  through  the  insulating 
substances  surrounding  the  conductors  in  the  useful  circuit,  and  the 
leakage  and  other  non-useful  circuits  follow  the  same  laws  as  the 
useful  circuit.  It  must  not  be  forgotten,  also,  that  the  algebraical 
sum  of  all  the  pressures  in  the  circuit  must  be  used  in  applying 
the  laws. 


DEFINITIONS,   UNITS,   ETC. 


7 


Series  and  Parallel  Circuits. — These  are  terms  that  will  be  met 
with  very  frequently  in  discussing  electrical  apparatus.  They  mean 
that  in  series  the  same  current  of  the  same  strength  passes  through 


LAMPS 


LAMPS 


FIG.  1. — Diagram  showing  Series-connections.    The  Lamps  shown  are  connected  in 
Series,  the  Current  passing  through  them  in  succession. 

each  apparatus  in  succession.  Each  apparatus  modifies  the  strength 
of  the  current  to  the  extent  of  its  own  resistance,  its  own  pressure  if 
it  brings  any,  and  its  own  back  pressure  if  it  creates  any;  but 
whatever  the  working  of  Ohm's  law  says  shall  be  the  current 
strength,  that  flows  through  the  whole  of  the  apparatus,  the 


1    AMD*; 

o    o    o  o    o 


FIG.  2.— Diagram  showing  Parallel-connections.    The  Lamps  shown  are  connected 
in  Parallel,  the  Current  passing  through  all  of  them  together. 

cables,  etc.,  connected  in  the  series.  Thus,  battery  cells  are  always 
connected  in  series,  the  zinc  pole  of  one  cell  being  connected  to 
the  carbon  pole  of  its  neighbour,  its  zinc  pole  to  the  carbon  of  the 


8 


ELECTRICITY  IN   MINING 


next,  and  so  on,  the  same  current  strength  passing  through  all  the 
batteries,  and  all  the  wires  and  apparatus  connected  to  them. 

In  parallel  working  the  current  is  divided  between  two  or  more 
branch  circuits,  which  are  said  to  be  connected  in  parallel.  It  is 
very  rare  that  a  number  of  branch  circuits,  or  parallels,  or  derivations 
are  connected  to  the  same  point,  as,  say,  the  terminals  of  a  generator ; 
but  it  is  very  common  for  a  number  of  parallels  to  be  connected  to  a 
pair  of  conductors,  such  as  a  pair  of  cables  that  are  connected  to  the 
terminals  of  a  generator.  A  familiar  instance  of  this  is  a  two-wire 
distribution  service  of  incandescent  lamps,  where  each  lamp  is 
bridged  between  the  cables,  the  separate  lamp  circuits  being  in 
parallel  with  each  other. 

Modifications  of  the  above  that  are  occasionally  employed  are 


LAMPS 


pIGt  3. — Diagram  showing  Series-parallel  or  Parallel-series  Connections.  The  two 
Lamps  in  each  Parallel  are  connected  in  Series,  and  the  Current  passes  through 
all  the  Parallels  together,  and  through  the  two  Lamps  in  each  Parallel  in 
succession. 

series-parallel,  and  parallel-series.  The  terms  mean  almost  the  same 
thing — two  or  more  apparatus  connected  together  in  series,  so  that  the 
same  current  passes  through  all  of  them,  and  the  different  series 
connected  in  parallel.  An  instance  of  this  is  the  case  of  two  in- 
candescent lamps  connected  in  series  across  a  two-wire  service  of 
double  the  pressure  the  lamps  are  designed  for.  Each  individual 
branch  has  its  lamps  in  series,  while  all  the  branches  are  in  parallel 
with  each  other.  Figs.  1,  2,  and  3  are  diagrams  of  the  connections 
of  series,  parallel,  and  series-parallel  services. 


Work  done  in  Electric  Circuits 

The  work  done,  or  the  rate  of  doing  work  in  electric  circuits,  is 
measured  by  the  product  of  the  pressure  between  the  ends  of  any 
conductor  or  group  of  conductors,  through  which  a  current  is  passing, 


DEFINITIONS,   UNITS,   ETC.  9 

multiplied  by  the  strength  of  the  current  passing.  It  is  important  to 
remember  that  the  pressure  must  be  taken  when  the  current  is 
actually  passing,  as  it  is  less  than  when  it  is  not  passing.  The 
formula  for  the  rate  of  doing  work  is — 

W  =  E  x  C 

or — 

W  =  C2  x  E 

or  again — 

F2 

W  =  — 
E 

the  second  and  third  equations  being  found  by  applying  the  value 
of  C  in  Ohm's  law  to  the  first.  In  the  above,  W  is  the  rate  of 
doing  work,  C  is  the  current  strength,  E  the  pressure,  and  E  the 
resistance ;  and  when  C  is  measured  in  amperes,  E  in  ohms,  and  E  in 
volts,  W  is  measured  in  watts.  The  watt  is  the  unit  of  the  rate  of 
doing  work,  and  is  equal  to  44'22  ft.-lbs.,  746  watts  representing 
1  H.P.  For  alternating  currents,  as  explained  on  p.  20,  the 
formulae  are  modified  by  the  addition  of  cos  ft,  and  become — 

W  =  E  x  C  x  cos  ft 
W  =  C2E  x  cos  ft 
and — 

w  __  E2E  cos  ft 
E 


Heating  Effect  of  Electric  Currents 

The  heating  effect,  or  the  heat  liberated  by  electric  currents,  is 
measured  by  the  same  formulae  as  for  work,  heat  being  one  form  of 
work,  but  H  is  used  in  place  of  W. 

Thus— 

H  = 
H  = 


E 

and  H  is  again  in  watts,  when  C  is  in  amperes,  E  in  volts,  and  E  in 
ohms ;  and  17'58  watts  equal  one  British  Thermal  Heat  Unit,  t  being 
the  time. 

The  Heat  Unit,  or,  as  it  is  written,  the  B.Th.  Unit,  is  that 
quantity  of  heat  that  will  raise  the  temperature  of  1  Ib.  of  pure  water 
1°  Fahr.,  the  water  being  at  32°. 

Specific  heat  is  somewhat  similar  to  specific  resistance ;  it  is  the 
ratio  between  the  quantity  of  heat  required  to  raise  1  Ib.  of  the 


io  ELECTRICITY   IN   MINING 

substance  1°  Fahr.  to  that  required  to  raise  1  Ib.  of  water  1°.  Most 
substances,  and  particularly  the  metals,  have  a  low  specific  heat,  this 
meaning  that  it  takes  a  smaller  quantity  of  heat  to  raise  their  tempe- 
ratures. It  will  be  seen  that  it  is  perfectly  practicable  to  calculate 
the  probable  increase  of  temperature  of  any  given  cable,  with  a  given 
current  or  pressure  applied  to  it,  for  a  given  time.  This  is  dealt  with 
more  fully  in  Chapter  V. 


Methods  of  producing  Electrical  Pressure 

There  are  four  methods  of  producing  or,  as  it  is  usually  termed, 
creating  electrical  pressure,  all  of  which  are  of  interest  to  the  mining 
engineer. 

1.  By  friction,  as   in  frictional   machines,  some   of  which   still 
survive  for  shot  firing,  in  which  a  glass  disc  is  rubbed  against  pads 
of  silk   or  other  substances.     Electricity  is  also  generated   by  the 
friction  of  steam  through  a  pipe,  that  of  a  belt  over  a  pulley,  and 
generally  wherever   two   substances   are  rubbed   together,  and   the 
conditions  are  such  that   the  electricity  generated  does  not  imme- 
diately get  away.     It  is   more   than   probable   that   the  enormous 
pressures  that  exist  between  different  clouds,  and   between  certain 
clouds  and  the  earth,  during  or  before  thunderstorms,  is  created  to 
a  large  extent  by  the  friction  of  the  body  of  the  cloud  against  the 
remainder  of  the  atmosphere. 

2.  By  chemical  action,  as  in  the  primary,  and  secondary  galvanic 
batteries,  or  accumulators,  as   the   latter  are  called.     An  electrical 
pressure  is  created  whenever  two   dissimilar  substances  come   into 
contact  with  each  other,  such  as  two  metals,  and  more  particularly 
when  two  dissimilar  metals,  or  a  metal  and  carbon,  are  present  in  a 
liquid,  an  electric  current  following  when  there  is  a  path  open  for  it. 
It  should  be  understood  that  it  is  by  no  means  necessary  that  the 
path  for  the  current  should  be  external  to  the  liquid.     In  the  very 
old  well-known  lecture  experiment,  where   a   zinc   and   copper   or 
carbon  plate  are  immersed  in  dilute  sulphuric  acid,  and  it  is  shown 
that   such  action  as  does  take  place  is  at  the  surface  of  the  zinc 
plate  when  no  connection  exists   between  the    plates,  while  when 
the  plates  are  connected  by  a  wire  outside  of  the  cell,  a  vigorous 
evolution   of  gas   takes   place   at   the   carbon  or  copper  plate,  the 
evolution  of  gas  will  take  place  equally  as  well   if  the  plates  are 
made  to  touch  each  other  under  the  surface  of  the  liquid.     This  is 
a  very  important  point  to  remember,  when  dealing  with  the  stray 
currents  that  are  often  met  with  from  electric  light  or  power  services, 
and  that  are  set  up  where  iron  and  copper,  or  copper  and  lead,  or 
other  metals,  are  together  in  the  presence  of  a  liquid.     It  is  supposed 


DEFINITIONS,   UNITS,   ETC.  n 

that  a  certain  pressure  must  exist  before  action  of  this  kind  can  take 
place,  and  before  the  electrolysis  which  follows,  and  which  is  often 
so  troublesome,  can  go  on.  As  a  matter  of  fact,  it  is  difficult  to  have 
a  difference  of  pressure  so  small  that  galvanic  action,  and  all  that  it 
means,  will  not  take  place  if  the  conditions  mentioned  above  are 
present. 

3.  By  magneto-electric  induction,  as  in  the  dynamo  machine,  or, 
as  it  may  be  perhaps  better  understood,  by  the  passage  of  conductors 
through   a  magnetic  field.     In   the   modern   dynamo  one   or  more 
powerful   magnetic  fields   are  arranged  within   a  cylindrical   space 
forming  the  centre  of  the  machine,  and  the  conductors  are  caused  to 
cut  the  lines  of  force  within  this  space,  either  by  themselves  being 
forced  through  the  magnetic  field  by  mechanical  power,  or  by  the 
electro-magnets,  as  will  be  explained  in  Chapter  IV.,  that  create  the 
magnetic   fields,  being   themselves   driven   through   the   cylindrical 
space  in  such  a  manner  as  to  change  the  strength  of  the  field  at  the 
points  where   the  conductors   are  placed.     Whenever  a  conductor 
passes   through   a  magnetic  field,   it  creates  an  electrical   pressure 
exactly  in  proportion  to  the  rate  at  which  it  is  driven  through  the 
field,  and  to  the  strength  of  the  field. 

4.  By  creating  a  difference  in  temperature  between  two  sets  of 
junctions  of  a  group  of  pairs  of  dissimilar  metals,  connected  in  one 
series  or  chain.     This  method,  known  as  thermo-electricity,  is  only 
of  use  for  measuring  temperatures ;  but  with  the  advance  of  science 
in  connection  with  mining,  and  particularly  with  the  greater  depths 
and   the   higher   temperatures   that   are   being   met   with,  and   the 
importance  of  obtaining  accurate  information  of  temperatures,  some- 
times at  a  distance  from   the  points  where  the  temperatures  exist, 
the  apparatus  will  probably  be  of  service.     Certain  metals,  bismuth 
and  antimony  notably,  if  plates  of  them  be  joined  together,  and  the 
junction  be  exposed   to   an  increase  of  temperature,  will   have   a 
difference  of  electrical  pressure  at  the  free  ends  of  the  two  plates. 
In  practice  a  number  of  pairs  of  plates  are  connected  together  in 
such    a  manner,   that  one   set    of  junctions   are   exposed   to    the 
temperature   it   is   desired   to   measure,  and  the   other   set   to   the 
ordinary  temperature  of  the  atmosphere,  or  any  temperature  that 
can  be  maintained  uniform  and  as  low  as  possible,  and  the  difference 
of  temperature  is  read  off  as  a  difference  of  pressure  upon  the  scale 
of  a  galvanometer. 

Magnetism  and  the  Magnetic  Circuit 

Modern  science  has  practically  accepted  Professor  Ewing's  theory 
of  magnetism.  Iron  and  steel,  and,  to  a  very  much  smaller  extent, 
nickel  and  cobalt,  are  conceived  to  have  their  molecules,  when  they 


12  ELECTRICITY  IN   MINING 

are  not  in  the  condition  we  call  magnetized,  forming  closed  figures. 
Ewing  supposes  that  each  molecule  is  a  tiny  needle  magnet,  having 
its  north  and  south  seeking  poles,  and  that  these  little  magnets  are 
arranged  in  groups  of  four  or  more,  in  such  a  manner  that  they 
satisfy  each  other's  attractions,  north-seeking  poles  and  south-seeking 
poles  lying  together.  When  what  we  call  a  source  of  magnetism 
arrives,  such  as  an  electric  current,  or  another  iron  or  steel  body  in 
the  state  we  know  as  magnetized,  the  closed  figures  open  under  the 
impelling  force,  the  little  needle  molecule  magnets  turning  all  in 
one  direction,  and  the  attractions  which  were  previously  satisfied  by 
the  arrangement  of  the  closed  figures  are  all  delivered  at  two  points 
or  surfaces,  known  respectively  as  the  north  and  south  poles.  The 
piece  of  iron  or  steel  that  has  been  subject  to  this  process  has  now 
acquired  certain  properties. 

1.  If  freely  suspended  it  will  endeavour  to  lie  in  the  earth's 
magnetic  meridian,  one  pole  turning  to  the  earth's  magnetic  north, 
and  the  other  to  the  earth's  magnetic  south ;  and  if  moved  over  the 
earth's  surface  it  will  tend  to  dip  towards  either  the  north  magnetic 
or  the  south  magnetic  pole,  according  as  it  is  in  the  neighbourhood 
of  either. 

2.  The  north-seeking  pole  of  every  magnetized  body  attracts  the 
south-seeking  pole  of  every  other  magnetized  body,  and  repels  the 
north-seeking  pole,  south-seeking  poles  repelling  south-seeking  and 
attracting  north-seeking. 

3.  When  a  magnetized  piece  of  iron  or  steel  is  brought  near  a 
piece  of  iron  or  steel  not  previously  magnetized,  it  induces  in  the 
latter  a  state  of  magnetization,  such  that  the  nearer  portions  become 
magnetized  in  the  opposite  sense  to  the  poles  of  the  inducing  body, 
this  leading  to  the  attraction  we  are  familiar  with  in  the  case  of 
electro-magnets  or  steel  magnets  for  iron. 

4.  When  the  condition  of  magnetization  has  been  created  in  any 
iron  or  in  any  mass  of  iron  or  steel,  what  are  termed  magnetic  lines 
of  force  are  sent  out  in  space  from  the  north-seeking  to  the  south- 
seeking  pole  of  the  body.     The  lines  of  force  assume  various  forms, 
according  to  the  body  from  which  they  emanate.     Where  the  two 
poles  of  the  magnetized  body  are  arranged  very  close  together  with 
a  small  space  between  them,  the  great  majority  of  the  lines  of  force 
pass   in  straight  lines  from  one   pole   to   the  other.     Where  other 
masses  of  iron  lie  in  the  path  of  the  lines  of  force,  they  modify  the 
curves  the  latter  take,  the  lines  passing  preferably  through  the  iron, 
and  resuming  curves   of  various  forms   after  leaving  it.     In   the 
dynamo  the  lines  of  force  pass  in  straight  lines  from  the  poles  of  the 
field  magnets  across  the  air  space  between  them  and  the  armature, 
and  in  more  or  less  curved  lines,  according  to  the  number  of  poles 
present,  as  will  be  explained  in  Chapter  IV.  in  the  iron  of  the 


DEFINITIONS,   UNITS,   ETC.  13 

armature.  The  lines  of  magnetic  force  behave  in  many  respects  like 
the  electric  current. 

There  is  a  magnetic  resistance,  or  reluctance,  as  it  is  sometimes 
called  by  preference ;  that  is,  all  bodies  offer  a  certain  resistance  to 
the  passage  of  the  lines  of  force  through  them.  Iron  and  steel  offer 
very  much  less  resistance  than  any  other  substances.  Air  and  the 
insulating  materials  that  are  used  in  connection  with  electrical 
apparatus  offer  practically  the  same  magnetic  resistance,  and, 
according  to  Professor  Kapp's  measurements,  1400  times  that  of  the 
resistance  offered  by  wrought  iron  or  mild  steel.  The  different 
qualities  of  iron  and  steel  offer  different  resistances,  cast  iron  having 
a  resistance  about  twice  that  of  the  best  wrought  iron  at  the  point 
of  saturation  usually  adopted.  The  mild  steel  that  has  been  intro- 
duced for  so  many  purposes  within  the  last  twenty-five  years,  has 
practically  the  same  magnetic  resistance  as  the  best  Swedish  wrought 
iron.  A  special  form  of  steel  made  at  Sheffield  by  Messrs.  Hadfield 
and  others  has  a  rather  lower  magnetic  reluctance  than  even  the 
best  Swedish  iron,  while  alloys  of  iron  and  nickel,  and  of  iron  and 
manganese,  have  very  high  reluctance.  For  nearly  the  whole  work  for 
which  magnetism  is  used,  excitation  is  by  means  of  electric  currents 
passing  in  wires,  coiled  round  the  mass  of  iron  to  be  magnetized ; 
and  there  is  a  definite  relation  between  the  strength  of  the  current 
passing  in  the  wires,  the  number  of  times  they  pass  round  the  body 
to  be  magnetized,  or  the  number  of  ampere  turns,  as  it  is  usual  to 
express  it,  and  the  number  of  lines  of  force  created  in  any  electro- 
magnetic system  with  any  given  magnetic  reluctance.  The  number 
of  ampere  turns  is  sometimes  called  the  magneto-motive  force,  and 
it  bears  the  same  relation  to  the  magnetic  reluctance,  made  up  of  the 
resistance  offered  by  the  iron  of  the  field  magnets,  etc.,  of  the  system, 
and  the  air  spaces,  as  the  electrical  pressure  does  to  the  resistance 
offered  by  conductors  and  insulators  to  the  passage  of  electricity. 

What  is  known  as  magnetic  permeability  is  the  ratio  between 
the  magnetic  flux  density,  the  number  of  lines  of  magnetic  force  per 
square  inch,  or  per  square  centimetre,  when  a  metal  is  present,  and 
that  when  only  air  is  present,  and  this  ratio  is  expressed  by  the 
Greek  letter  p.  It  is  usual  to  express  a  flux  density  that  would  be 
produced  in  air  with  a  given  number  of  ampere  turns  by  the  letter 
H,  the  number  that  is  produced  in  a  given  specimen  of  iron  by 
the  letter  B,  and  the  ratio  between  B  and  H,  which  expresses  the 
relative  permeability  of  the  specimen,  by  the  Greek  letter  p.  The 
permeability  varies  from  0*999  for  bismuth  up  to  4000  for  special 
qualities  of  Swedish  iron  and  magnetic  steel. 

In  the  dynamo,  and  in  every  electro-magnetic  apparatus,  there 
is  a  magnetic  circuit,  built  up  of  the  iron  cores  of  the  electro- 
magnets, the  iron  yoke  connecting  them  at  the  back,  and  the 


i4  ELECTRICITY   IN   MINING 

iron  armature  which,  with  the  air  space  between  the  magnetic  pole 
and  the  armature,  completes  the  circuit.  The  simplest  case,  and 
one  which  shows  the  magnetic  circuit  in  its  simplest  form,  is 
that  commonly  employed  for  electric  bells  and  similar  apparatus. 
Commencing  from  the  north-seeking  pole  of  the  electro-magnet, 
the  lines  of  force  pass  across  the  air  space  to  the  iron  armature 
in  front  of  the  poles,  through  the  iron  armature  to  the  air  space 
opposite  the  south-seeking  pole,  across  the  air  space  to  the  south- 
seeking  pole,  through  the  iron  core  to  the  yoke,  through  the  yoke 
to  the  other  leg  of  the  magnet,  down  the  other  leg  to  the  north- 
seeking  pole.  In  the  dynamo  there  may  be,  and  in  the  modern 
dynamo  usually  are,  several  magnetic  circuits,  but  the  lines  of  force 
pass  in  exactly  the  same  manner,  from  the  north-seeking  pole  across 
the  air  space  to  the  armature,  through  a  portion  of  the  armature,  across 
the  air  space,  and  through  the  magnet  and  its  yoke. 

The  larger  the  cross-section  of  the  magnet  cores  and  the  shorter 
their  length,  the  lower  is  the  magnetic  reluctance  of  that  part  of 
the  magnetic  circuit ;  and  the  shorter  the  distance  between  their 
magnetic  pole  and  the  armature,  and  the  larger  the  area  of  the 
armature  embraced  by  the  magnetic  pole,  the  lower  is  the  resistance 
of  the  important  portion,  the  air  space. 


Difference  between  Alternating  and 
Continuous  Currents 

The  apparatus  that  have  been  described  uses  what  have  been 
termed,  in  contra-distinction  to  the  alternating  form,  continuous 
currents.  The  continuous  current  passes  always  in  the  same  direc- 
tion. Whether  it  be  a  wave  motion  or  a  procession  of  charged 
corpuscles,  it  commences  from  one  pole  of  the  generator  called  the 
positive,  passes  through  the  wires,  cables,  etc.,  connecting  the 
generator  with  the  apparatus  to  be  worked,  through  the  apparatus 
to  be  worked,  the  cables,  etc.,  completing  the  circuit,  back  to  the 
generator,  and  through  the  generator  to  the  positive  pole  from  which 
it  set  out.  Alternating  currents  are  constantly  changing  in  direc- 
tion and  in  value.  Thus,  a  current  will  set  out  from  one  pole  of  the 
generator  which  for  the  moment  is  the  positive,  will  pass  through 
the  cables  and  the  apparatus  to  be  worked,  back  to  the  other  pole 
of  the  generator  which  for  the  moment  is  the  negative  pole,  and 
through  the  generator  to  the  pole  it  set  out  from.  Then  a  current 
in  the  opposite  direction  sets  out  from  the  pole  which  was  the 
negative  with  the  first  current,  and  passes  through  the  circuit  and 
the  generator  in  the  opposite  direction  to  the  first.  This  is  suc- 
ceeded by  another  current  in  the  first  direction,  and  so  on,  these 


DEFINITIONS,   UNITS,   ETC.  15 

opposite  currents  succeeding  each  other  at  from  fifty  times  a  second 
upwards.  In  addition  to  this,  the  currents  are  constantly  changing 
in  value.  The  first  current  which  commences  is  at  first  very  weak, 
it  gradually  increases  to  a  maximum,  then  gradually  falls  to  zero, 
and  it  is  when  it  has  fallen  to  zero  that  it  is  succeeded  by  the 
second  current  in  the  opposite  direction,  which  also  commences 
from  nothing,  rises  gradually  to  a  maximum,  falls  to  zero,  and  is 
again  succeeded  by  a  reverse  current.  The  laws  governing  the 
working  of  circuits  in  which  alternating  currents  are  used  are  the 
same  as  the  laws  governing  those  in  which  continuous  currents  are 
used,  but  certain  additions  have  to  be  made  to  the  equations  employed, 
and  certain  values  have  to  be  taken  for  the  pressures  and  currents. 

Virtual  or  Effective  Volts  and  Amperes.— For  the  calculations 
that  are  employed  with  alternating  currents  for  Ohm's  law,  etc.,  what 
are  termed  virtual  or  effective  pressures,  and  currents  are  used.  They 
are  the  pressures  and  currents  which  would  perform  the  equivalent 
heating,  if  continuous  currents  were  employed.  What  is  required  is 
a  certain  average  pressure  and  average  current,  but  the  ordinary  rule 
of  averages  will  not  apply,  because  the  currents  and  pressures  do  not 
increase  in  the  regular  proportion,  but  in  that  in  which  the  values  of 
the  sine  of  an  angle  ranging  between  0  and  90°  increase,  and  it  is  an 
average  obtained  from  the  changes  in  the  value  of  the  sines.  Stated 
accurately,  the  effective  or  virtual  pressure  or  current  is  the  square 
root  of  the  mean  of  all  the  values  of  the  pressures  and  currents 
throughout  a  half -period.  A  complete  cycle  or  period  consists 
of  the  increase  from  zero  to  maximum  in  one  direction,  the  fall  to 
zero,  the  increase  to  the  maximum  in  the  opposite  direction,  and  the 
fall  to  zero  again.  The  rule  for  the  effective  values  is  based  upon 
the  fact  that  the  heating  value  of  any  current,  and  of  any  pressure, 
with  the  resistance  of  the  conductor  constant,  varies  as  the  square 
of  the  current,  and  as  the  square  of  the  pressure  delivered  to  the 
conductor.  The  mean  required  is  the  mean  of  the  heating  effects 
of  all  the  values  assumed  by  the  pressure  and  the  current,  and 
obviously  the  square  root  of  this  sum,  whatever  it  may  be,  will  be 
the  effective  working  or  virtual  pressure  and  current  to  be  used  in 
calculations.  Put  in  another  way,  the  effective  pressure  is  that 
which  enables  makers  of  incandescent  lamps  to  label  their  lamps 
for  any  pressure,  which  is  applicable  to  continuous  or  alternating 
current  supply.  The  effective  pressure  and  the  effective  current 
are  both  0'707  of  the  maximum  pressure  and  current  reached  in 
either  the  positive  or  negative  direction  ;  or,  put  the  other  way,  the 
maximum  pressure  and  the  maximum  current  in  either  direction  are 
1*414  times  the  effective  or  working  pressure  and  current.  One 
important  effect  of  this  will  be  seen  from  the  fact  that  when  we  talk 
of  an  alternating  current  of  100  or  1000  volts,  we  actually  have 


16  ELECTRICITY  IN   MINING 

pressures  at  different  instants,  141*4  volts  in  the  one  case,  and  1414 
volts  in  the  other  case  of  opposite  name ;  or  a  difference  of  pressures 
at  successive  instants,  half  a  period  apart,  of  282*8  and  2828  volts 
respectively. 


Two- Phase  and  Three- Phase  Circuits 

The  alternating  current  described  on  p.  14  is  known  as  the  single- 
phase  current.  In  France  it  is  usually  called  the  simple  alternat- 
ing current.  Both  terms  are  employed  to  distinguish  it  from 
two,  three,  and  poly  phase  currents.  Two-phase  currents  are  merely 
two  single-phase  currents,  usually  generated  in  one  machine,  which 
follow  each  other,  separated  in  time  by  a  quarter  of  a  period. 
The  currents  of  the  two  phases  have  the  same  periodicity.  If  fifty 
periods  is  the  frequency,  each  current  completes  its  cycle,  rising  and 
falling  and  reversing  in  one- fiftieth  part  of  a  second.  But  the  second 
current  does  not  commence  to  rise  on  its  positive  side  until  the  first 
current  has  passed  through  a  quarter  of  its  cycle.  That  is  to  say,  the 
second  current  is  at  its  zero,  commencing  its  ascent  to  its  positive 
maximum  when  the  first  current  has  reached  its  positive  maximum. 
When  the  second  current  reaches  its  positive  maximum,  the  first 
current  has  reached  its  second  zero.  When  the  second  current 
reaches  its  second  zero,  the  first  current  has  reached  its  negative 
maximum,  and  so  on,  the  two  currents  being  always  separated  by  a 
quarter  of  a  period,  or  by  an  angle  of  90° ;  or,  as  it  is  often  expressed, 
they  are  in  quadrature. 

Three-phase  currents  are  merely  single-phase  currents  succeed- 
ing each  other  in  the  same  manner  as  the  two  currents  of  a  two-phase 
service  do,  but  the  currents  are  separated  by  an  interval  in  time  of 
one-third  of  the  total  period  of  the  single-phase  current.  That  is  to 
say,  with  three-phase  currents  the  second  current  commences  to  rise 
when  the  first  current  has  completed  one-third  of  its  cycle ;  that  is, 
when  it  has  reached  its  maximum,  and  accomplished  one-third  of  its 
descent  to  its  second  zero.  When  the  second  current  reaches  its 
positive  maximum,  the  first  current  has  passed  through  its  second 
zero,  and  is  one-third  on  its  way  to  its  negative  maximum,  and  so  on, 
the  two  currents  being  always  one-third  of  a  period  or  120°  apart. 
The  third  current  commences  when  the  second  current  has  accomplished 
one-third  of  its  cycle,  and  when  the  first  current  has  accomplished 
two-thirds  of  its  cycle.  When  the  third  current  is  at  its  first  zero, 
and  about  to  commence  its  first  ascent  to  its  positive  maximum,  the 
second  current  will  be  passed  through  its  positive  maximum,  and  be 
one-third  the  way  down  to  its  second  zero,  while  the  first  current 
will  be  passed  through  its  second  zero,  and  be  two-thirds  on  its  way 


DEFINITIONS,   UNITS,   ETC.  17 

to  its  negative  maximum.  The  third  current  follows  the  second 
current  in  the  same  manner  as  the  second  current  follows  the  first, 
the  three  currents  always  being  separated  by  one-third  of  a  period  in 
time  and  120°. 

In  America,  four-phase  and  six-phase  currents  are  sometimes 
employed,  particularly  six-phase,  though  they  have  not  hitherto  been 
used  in  this  country. 

With  four-phase  currents  there  are  merely  four  single-phase 
currents,  separated  in  time  by  a  quarter  of  a  period,  and  succeeding 
each  other  at  the  different  maxima  and  zeroes  by  this  amount.  With 
six-phase  currents  there  are  six  single-phase  currents,  succeeding 
each  other  at  intervals  of  one-sixth  of  a  period  of  the  cycle,  and 
being  always  separated  by  that  amount  and  by  60°. 


Electrostatic  Induction  and  the  Capacity  of 

Cables 

The  electrostatic  capacity  of  cables  has  only  recently  become 
of  importance  in  mining  work,  and  only  owing  to  the  fact  that 
difficulty  arose  in  measuring  the  leakage  currents  on  three  phase 
services,  because  the  capacity  currents  apparently  masked  the  leakage 
current. 

When  an  electrical  pressure  is  applied  to  any  cable,  and  more 
particularly  when  the  cable  is  either  lead  covered  or  armoured,  before 
the  current  which  the  pressure  causes  to  pass  through  the  cable  can 
reach  the  end  of  the  cable,  an  electrostatic  charge  has  to  be  delivered 
to  the  insulating  envelope  of  the  cable.  The  insulating  envelope, 
whether  it  be  indiarubber,  bitumen,  paper,  or  fibre,  has  what  is 
termed  an  electrostatic  capacity ;  that  is  to  say,  a  capacity  for  absorb- 
ing a  certain  quantity  of  electricity  in  the  same  manner  as  the  Leyden 
jars,  with  which  we  are  so  familiar  on  lecture  tables,  do.  The  insu- 
lating envelope  may  be  likened  to  a  sponge  into  which  the  electricity 
soaks,  and  the  current  that  is  to  perform  work  at  the  end  of  the  cable 
cannot  pass  on  until  each  length  of  the  insulator — each  foot,  inch, 
yard,  etc. — has  soaked  to  its  full  capacity. 

The  quantity  of  electricity  the  insulator  will  absorb  depends 
directly  upon  the  pressure  between  the  conductor  and  the  lead 
covering  or  armour,  or  whatever  the  outside  of  the  cable  may  be  in 
contact  with  ;  it  depends  also  directly  upon  what  is  termed  the 
specific  inductive  capacity  of  the  material  itself,  that  of  indiarubber 
being  2*34  to  2*94,  and  of  the  impregnated  paper  used  for  the 
insulation  of  paper-covered  cables  2*5. 

It  also  depends  directly  upon  the  extent  of  the  surfaces  of  the 
conductor,  and  of  the  armour  or  lead  covering,  or  other  substance  on 

c 


1 8  ELECTRICITY  IN   MINING 

the  outside  of  the  cables,  that  are  opposed  to  each  other,  and  inversely 
upon  the  thickness  of  the  insulating  envelope.     It  will  be  seen  that 
it  is  an  advantage  in  this  matter  to  have  a  thick  insulating  envelope, 
and  that  paper-covered  cables  have  an  advantage  over  both  india- 
rubber  and  bitumen  covered.     It  will  be  seen,  also,  that  long  cables, 
such  as  are  often  used  in  mines,  may  have  a  comparatively  large 
electrostatic  charge ;  and,  further,  that  the  cables  employed  with  three 
phase  work  are  under  the  best  conditions  for  absorbing  a  compara- 
tively large  charge.     When  the  pressure  is  removed,  as  when  the 
circuit  is  opened,  the  charge  which  is  held  in  the  cable  is  released, 
and  immediately  commences  to  flow  back  into  the  conductor  from 
which  it  was  received.     A   decrease  of  pressure  operates  in  the 
same  manner,  though  to  a  smaller  degree  than  a  complete  cessation 
of  the  pressure.     Also,  an  increase  of  pressure  operates  exactly  as 
a  newly  arrived  pressure.    With  alternating  currents,  the  pressures 
are    continually  rising  and   falling  and  reversing,  and  therefore  in 
cables  that  are  employed  for  alternating  current  services,  the  con- 
denser,  as   it   is   called,   formed   by   the   conductor,    its   insulating 
envelope  and  the  external  conductors,  is  being  continually  charged, 
discharged,  and  recharged  in  the  opposite  direction,  the  result  being 
that  there  is  an  absorption  of  a  certain  quantity  of  electrical  energy 
by  the  insulator  of  the  cables. 

The  returning  current  from  the  condenser  gives  rise  to  an  electrical 
pressure  in  the  conductor,  not  actually  in  opposition  to  the  pressure 
creating  the  current  from  which  the  original  condenser  charge  is 
obtained,  but  in  quadrature  with  it.  That  is  to  say,  the  pressure 
created  by  the  condenser  action  occupies  the  same  position  with 
reference  to  the  pressure  creating  the  current  normally  in  the  con- 
ductor, as  the  two  currents  in  a  two  phase  service  do  to  each  other. 
The  pressure  created  by  the  condenser  action  has  what  is  known  as  a 
lead.  After  the  service  has  been  in  operation  a  sufficient  time  to 
charge  the  condenser,  and  to  allow  it  to  discharge,  the  pressure 
created  by  the  condenser  is  in  the  position  that  would  be  represented 
by  a  current  90°  in  advance. 

The  condenser  pressure  opposes  that  created  by  electro-magnetic 
induction,  as  described  on  p.  19,  and  if  the  two  can  be  made  to 
balance,  the  primary  pressure  is  less  than  it  otherwise  would  be. 
If  it  is  not  balanced,  an  additional  pressure  has  to  be  added  to  the 
ordinary  pressure  than  would  be  necessary  to  drive  a  given  current 
through  the  system  of  conductors  forming  the  service.  The  condenser 
pressure  is,  however,  usually  of  very  little  importance. 


DEFINITIONS,   UNITS,   ETC.  19 


Electro-Magnetic  Induction 

In  addition  to  charging  the  electrical  condenser,  of  which  the 
conductor  of  a  cable  forms  a  part,  the  current  which  passes  through 
any  conductor  under  the  influence  of  an  electrical  pressure  has  to 
create  an  electro-magnetic  field  around  each  unit,  each  inch  or  yard 
of  the  conductor,  before  it  can  pass  on  to  the  next  inch,  yard,  etc. 
With  continuous  currents  this  phenomena,  as  also  that  of  the  con- 
denser charge,  merely  delays  for  a  very  inappreciable  time  the  first 
passage  of  the  current  through  the  conductors,  and  the  apparatus  they 
are  connected  with.  But  with  alternating  currents,  which  are  rising 
and  falling  and  reversing  constantly,  there  is  a  constant  creation  of  an 
electro-magnetic  field  round  the  conductor,  and  a  constant  delivery  of 
the  energy  which  created  the  field,  to  the  conductor  from  which  it 
was  taken,  a  constant  recreation  of  the  field  in  the  opposite  sense, 
followed  by  a  redelivery  to  the  conductor,  and  so  on.  This  leads  to 
what  is  called  the  lag  of  the  current  behind  the  pressure  which 
creates  it. 

Even  with  alternating  currents  the  lag  is  not  of  importance  with  a 
straight  cable,  unless  the  cable  is  very  long,  and  the  periodicity  of  the 
current  is  very  high.  But  in  all  dynamo  machines,  whether  con- 
structed for  generating  current,  or  for  converting  current  into 
mechanical  power,  the  conductors  which  create  the  magnetic  fields  in 
the  machines  are  coiled  on  themselves  a  very  large  number  of  times, 
and  the  variation  of  the  current  in  each  coil  acts  upon  all  the  turns  of 
the  conductor,  the  result  being  that  the  total  inductive  effect,  the 
self-induction  as  it  is  termed,  is  multiplied  very  considerably,  and 
with  alternating  current  dynamos  and  motors,  the  lag  caused  by  the 
electro-magnetic  induction  is  often  very  considerable. 

The  reduction  creates  a  pressure  acting  at  right  angles  to  the 
pressure  of  the  service.  The  pressure  created  by  electro -magnetic 
self-induction  is  in  direct  opposition  to  the  pressure  created  by  the 
electrostatic  induction.  The  electro-magnetic  induction  is,  however, 
by  far  the  most  important  of  the  two,  and  it  usually  leads  to  the 
necessity  of  modifying  the  formula  for  calculating  the  power  expended 
in  any  electrical  circuit,  by  the  addition  of  a  factor  allowing  for  the 
current  not  acting  at  the  same  instant  as  the  pressure.  With  the 
great  majority  of  alternating  current  circuits,  especially  where  there 
are  a  number  of  motors  in  use,  the  current  lags  considerably  behind 
the  pressure  that  creates  it,  owing  to  the  electro-magnetic  induction 
referred  to,  and  only  a  certain  portion  of  the  current  can  be  taken  as 
acting  at  the  same  instant  as  the  pressure.  This  factor  is  pro- 
portional to  the  cosine  of  the  angle  by  which  the  current  lags  behind 
the  pressure.  If  the  pressure  be  represented  by  a  radius  sweeping 


20  ELECTRICITY   IN   MINING 

out  an  angle  round  a  centre,  the  current  is  represented  by  a  second 
radius  a  certain  number  of  degrees  behind  the  first,  and  it  is  the 
cosine  of  the  angle  between  the  two  that  represents  the  additional 
factor  that  has  to  be  applied  to  the  ordinary  power  formula.  The 
angle  is  termed  0,  and  cosine  0  is  called  the  power  factor.  Keference 
to  a  table  of  cosines  will  show  that  when  the  angle  is  0,  the  cosine 
=  1,  and  when  the  angle  is  90,  the  cosine  =  0. 

In  practice  it  is  usual  to  take  the  value  of  cosine  <t>  for  calcula- 
tions as  0'8,  but  it  may  have  as  low  a  value  as  0*5,  and  with  what 
are  termed  non-inductive  circuits,  circuits  in  which  only  incande- 
scent lamps  are  employed,  and  in  which  there  are  no  electro-magnets 
or  other  arrangements  of  wires  coiled  on  each  other,  it  is  usually  1, 
or  nearly  so. 

The  Impressed  Pressure  in  Inductive  Circuits.— The  presence 
of  electro-magnetic  and  electrostatic  induction  obliges  a  higher 
pressure  to  be  maintained  by  the  generator  than  would  otherwise  be 
necessary,  unless  the  two  inductive  actions  neutralize  each  other. 
The  additional  pressure  required,  or  the  total  impressed  pressure  or 
impressed  voltage,  as  it  is  termed,  is  found  by  a  simple  adaptation 
of  the  parallelogram  of  forces  employed  in  mechanics.  The  pressure 
creating  the  current  and  the  induced  pressures  are  at  right  angles  to 
each  other,  as  explained.  If,  then,  the  two  are  set  out  in  the  manner 
of  the  parallelogram  of  forces,  the  original  pressure  demanded  by  the 
ohmic  resistance  of  the  circuit  forming  one  side  of  the  parallelogram, 
the  net  induced  pressure  forming  another  side,  and  the  parallelogram 
be  completed,  the  diagonal  of  the  parallelogram,  which  is  also  the 
hypothenuse  of  the  triangle  formed  by  the  original  current  pressure — 
the  C2E  pressure,  as  it  is  often  expressed — and  the  induced  pressure, 
gives  the  value  of  the  impressed  pressure,  and  it  may  be  measured 
graphically  by  scale,  or  calculated  by  using  the  47th  proposition  of 
the  first  book  of  Euclid,  the  square  root  of  the  impressed  pressure  = 
the  square  root  of  the  sum  of  the  square  of  the  current  and  induced 
pressures ;  or,  put  into  a  formula — 

E,2  =  Ec2  +  Ea2 


or  E;  =  \/Ec2  +  Ea2, 

where  Ef  is   the   impressed   pressure,  EC  that  due  to  the  current, 
and  Ea  that  due  to  induction. 


Electrolysis 

Electrolysis  is  the  phenomena  which  operates  in  galvanic 
batteries,  both  primary  and  secondary,  causing  the  liquid  electrolyte 
to  be  split  up  into  its  components,  one  body  of  the  components,  the 


DEFINITIONS,   UNITS,   ETC.  21 

non-metals,  appearing  at  the  anode,  the  plate  where  the  current 
enters  the  liquid,  and  the  other  the  metals  and  hydrogen  gas,  which 
behaves  as  a  metal  in  this  case,  appearing  at  the  cathode,  the  plate 
where  the  current  leaves  the  liquid.  In  the  Leclanche  battery  the 
electrolytic  action  splits  up  the  water  and  the  chloride  of  ammonium 
(sal  ammoniac)  into  its  components,  the  oxygen  and  chlorine  gases 
appearing  at  the  zinc  plate,  and  the  hydrogen  and  ammonia  gases 
appearing  at  the  carbon  plate.  In  this  case  the  hypothetical  metal 
ammonium  is  delivered  at  the  carbon  plate,  and  is  split  up  into 
ammonia  gas  and  hydrogen  gas.  It  is  the  hydrogen  gas  which  gives 
so  much  trouble  in  the  primary  galvanic  battery.  When  it  is 
delivered  at  the  carbon  plate  it  sets  up  a  pressure  opposing  that  of 
the  primary  current  which  created  it.  In  the  secondary  battery  or 
accumulator,  the  solution  of  sulphuric  acid  is  split  up  into  the  acid 
radicle  864  and  hydrogen,  the  acid  radicle  being  again  split  up,  and 
this  being  the  source  of  the  oxygen  on  the  one  hand,  which  oxidizes 
the  active  material  on  the  positive  plate  to  a  higher  oxide,  and  of  the 
hydrogen,  which  reduces  the  active  material  on  the  negative  plate  to 
the  metallic  state.  But  electrolysis  has  a  very  much  more  im- 
portant bearing  upon  the  use  of  electricity  in  mining,  than  merely 
the  part  it  plays  in  batteries.  Wherever  a  current  passes  through  a 
liquid,  no  matter  how  small  the  pressure  may  be,  and  no  matter  how 
small  the  surfaces  of  the  metals  from  and  to  which  it  passes,  nor, 
again,  how  small  the  quantity  of  liquid  through  which  it  passes  may 
be,  electrolysis  takes  place,  the  liquid  being  split  up  as  explained ; 
and  as  all  liquids  contain  oxygen  and  hydrogen,  oxidation  of  the 
surface  of  the  metal  from  which  the  current  passes  also  always  takes 
place.  This  leads  in  many  cases  to  metals  about  the  mine  being 
eaten  away,  sometimes  in  a  mysterious  manner.  Under  the  very 
best  conditions  leakage  always  takes  place  from  the  lighting  and 
power  service,  and  the  leakage  current  finds  its  way  by  every  path 
that  is  open  to  it  back  to  the  machine ;  and  it  often  happens  that  the 
path  includes  metals,  such  as  iron  rails,  ropes,  pulleys,  and  so  on,  and 
wherever  the  current  passes  from  a  metal,  it  invariably  attacks  the 
metal.  The  current  may  be  small,  but  it  is  always  acting,  and  the 
results  may  be  serious. 


CHAPTER   II 


ELECTRIC   MINING   SIGNALS   AND 
TELEPHONES 

Electric  Mining  Signals 

THERE  are  only  two  forms  of  signal  employed  now  in  mines,  those 
for  signalling  from  any  part  of  engine  roads  to  the  engine-man  and 
to  the  hooking-on  stations,  and  those  for  working  the  cages  in  the 
shaft.  Shaft  signals  have  not  even  yet  been  much  adopted  in 
English  mines,  but  engine-road  signals  are  almost  universal. 

The  Engine-road  Signal.  —  In  the  engine-road  signal  there  are 
two  or  three  naked  galvanized  iron  wires  stretched  tight  to  the  full 


-II 


CENTIAMMCTCR 


FIG.  4. — Diagram  of  Two-wire  Engine-road  Signal,  with  one  Bell,  Battery, 
Switch,  and  Centiammeter. 

length  of  the  engine  road,  and  secured  to  insulators  which  are  held 
by  the  props  on  the  side  of  the  road,  or,  where  that  is  not  convenient, 

22 


ELECTRIC   MINING  SIGNALS   AND   TELEPHONES     23 

by  the  beams,  or  again  by  stout  plugs  driven  into  holes  drilled  in  the 
coal  or  rock.  In  the  simplest  form  of  signal  there  are  two  wires,  one 
bell,  and  a  battery  in  the  engine-house,  and  a  circuit  is  formed,  as 
shown  in  Fig.  4,  so  that  when  connection  is  made  between  the  two 
naked  iron  wires  at  any  part  of  the  engine-road,  the  circuit  is 
completed,  and  the  bell  rings.  An  extension  of  this  is  the  signal 
working  to  an  engine-house  at  bank,  with  a  repeating  bell  at  the  pit 
bottom.  In  this  case  the  bell  and  battery  are  in  the  engine  house  at 
bank.  There  are  two  insulated  copper  wires  in  the  shaft,  and  they 
are  connected  to  the  naked  iron  wires  in  the  engine  road  in  such  a 


BELL 


FIG.  5.— Diagram  of  Two-wire  Engine-road  Signal,  as  arranged  for  signalling  to 
the  Shaft-bottom  and  Engine-house  simultaneously,  with  two  Bells,  one 
Battery,  Switch,  and  Centiammeter. 

manner  that,  as  shown  in  Fig.  5,  the  two  bells  are  included  in  the 
circuit  when  connection  is  made  between  the  iron  wires.  Another 
extension  of  the  signal  is  shown  in  Fig.  6,  where  there  is  a  bell  and 
battery  in  the  engine-house,  and  a  second  bell  at  the  end  of  the 
engine  road  with  three  wires,  connection  between  two  of  which  com- 
pletes the  circuit  and  rings  both  bells.  A  further  extension,  and 
one  that  has  come  very  much  into  use  since  the  general  adoption  of 
the  endless  rope  system,  is,  there  is  a  bell  in  the  engine-house  and  a 
battery  as  before,  a  bell  at  the  end  of  the  road,  and  one  at  each  of 


24  ELECTRICITY  IN   MINING 

the  hooking-on  stations.  There  are  three  wires,  and  connection 
between  two  of  them  causes  all  of  the  bells  to  ring  together.  The 
bells  may  be  connected  either  in  series  or  in  parallel,  as  shown  in 
Figs.  7  and  8.  The  series  system  requires  the  largest  number  of 
cells  in  the  battery  ;  the  parallel  system  requires  that  there  shall  be 
the  largest  individual  cells  in  the  battery,  if  the  signal  is  to  keep  up 
to  its  work,  as  it  makes  the  largest  drain.  The  parallel  system  also 
requires  that  the  bells  shall  be  exactly  alike  in  construction,  and 
especially  that  the  resistance  of  the  bell  coils  shall  be  exactly  alike. 
The  resistance  of  the  bell  coils  is  higher  than  would  be  necessary 
with  the  series  system  so  as  to  allow  for  the  difference  in  the 
pressure  at  the  bells  farthest  from  the  battery,  If  one  of  the  bells 

BELL  BELL 


SWITCH 


BATTERY 


CCNTlAMMETER 

FIG.  6. — Diagram  of  Three-wire  Engine-road  Signal,  with  Bells  at  each 
end  of  the  Road,  one  Battery,  Switch,  and  Centiammeter. 

has  a  lower  resistance  than  the  others,  it  will  take  more  current  than 
they  do,  and  may  lead  to  the  others  not  working,  especially  when 
the  battery  works  down. 

The  Willis  Engine-road  Indicator  Signal. — This  is  a  further 
development  of  the  engine-road  signal  that  has  been  worked  out  by 
Mr.  Willis,  of  the  Lycett  Collieries  in  North  Staffordshire,  and  applied 
there  with  success.  A  difficulty  is  sometimes  met  with  in  letting 
the  engine-man  know  what  portion  of  the  road  has  stopped  him. 
The  usual  practice  is  one  rap  on  the  bell  is  given  for  "  stop,"  two  or 
three  raps  for  "  go  on."  With  the  endless  rope  system  trams  are 
being  hooked  on  at  all  parts  of  the  road  at  all  times,  and  the  rope  is 


ELECTRIC   MINING  SIGNALS   AND   TELEPHONES     25 

required  to  be  stopped  only  when  the  hooker-on  has  trouble  with 
his   tram.      It    may  and  does    often   happen    that    two    or   more 


BELLS 


BELLS 


SWITCH 


BATTERY 


CENTIAMMETER 


FIG.  7.  —  Diagram  of  a  Three-wire  Engine-road  Signal  for  Endless  Rope  Haulage, 
with  Bells  at  each  Branch  Road,  the  Bells  being  connected  in  Series,  with  one 
Battery,  Switch,  and  Centiammeter. 

hookers-on  rap  for  "  stop  "  at  the  same  time,  or  in  immediate  suc- 
cession, and  the  man  who  gets  his  tub  clear  first  will  rap  "  go  on," 


BCLL 


a 


&CLL 


BELL 


BELL 

^ 


BATTERY 


CENTIAMMETER 

FIG.  8.— Diagram  of  Three-wire  Endless  Rope  Haulage  Signal,  similar  to 
Fig.  7,  but  with  the  Bells  connected  in  Parallel. 

not  knowing  that  the  other  man  is  not  ready.     If  the  engine-man 
goes  on  he  may  cause  damage  to  the  tram,  and  possibly  to  the  man 


26  ELECTRICITY   IN   MINING 

who  is  handling  it.  The  Willis  system  is  intended  to  prevent  this 
by  showing  the  engine-man  which  section  has  stopped  him,  and  then 
he  will  not  go  on  again,  no  matter  who  else  signals  him,  until  he 
receives  the  signal  from  each  one  that  has  signalled  "  stop."  Mr. 
Willis  divides  the  road  into  a  certain  number  of  sections,  presumably 
according  to  its  length.  A  road  is  divided  into  six  sections,  as 
arranged  in  a  set  of  signals  in  practical  use  at  Lycett.  The  naked 
iron  wires  that  were  described  in  connection  with  engine-road 
signals  generally  are  employed ;  but  one  of  them  is  broken  at  each 
junction  between  successive  sections,  and  at  the  break  an  electrical 
resistance  is  inserted,  which  is  also  made  to  do  duty  as  a  relay,  as 
will  be  explained.  It  will  be  seen  that  in  the  first  section  of  the 
road,  that  nearest  to  the  engine-house,  the  only  resistance  in  the 
circuit,  when  a  signal  is  given,  is  that  of  the  two  naked  iron  wires, 
and  that  of  the  apparatus  in  the  engine-house.  In  the  second 
section  there  will  be  the  resistance  of  the  iron  wires  of  the  first 
section,  that  of  the  resistance  interposed  between  the  two  sections, 
and  whatever  portion  of  the  iron  wires  in  the  second  section  may 
be  included  up  to  the  signalling  point.  In  the  third  section  there 
will  be  two  additional  resistances  besides  the  iron  wires,  and  so  on. 
In  the  engine-house  there  are  as  many  relays  as  there  are  sections. 
The  relay  is  described  on  p.  36.  It  is,  shortly,  an  electro- 
magnet, through  which  a  small  current  is  intended  to  pass,  and 
whose  armature  is  intended  to  close  a  second  circuit,  bringing 
into  operation  a  larger  current  with  bell  and  battery,  or  whatever 
may  be  arranged.  In  addition  to  this  usual  arrangement,  Mr.  Willis 
causes  the  armatures  of  his  relays  in  the  engine-house  to  perform  a 
double  office.  The  relays  are  all  connected  in  series,  the  current 
passing  directly  to  the  coils  of  the  first  relay,  thence  to  the  armature 
of  its  relay,  and  from  the  back  stop  of  the  first  relay  to  the  coils  of 
the  second  relay,  from  thence  to  its  armature,  and  from  its  back 
stop  to  the  coils  of  the  third  relay,  the  back  stop  of  the  third  relay 
to  the  coils  of  the  fourth  relay,  and  so  on.  So  that  if  the  armature 
of  any  relay  of  the  series  is  pulled  up  to  its  electro-magnet,  the 
circuits  of  all  the  relays  behind  it  are  broken,  and  no  current  passes 
to  them.  Thus,  if  the  first  relay  is  actuated,  the  second,  third, 
fourth,  fifth,  and  sixth  relays  are  all  dead.  There  is  a  separate  relay 
for  the  bell,  actuated  at  the  same  time  as  each  one  of  the  other  relays, 
and  working  with  them.  The  different  relays  are  arranged  to 
respond  to  the  current  from  the  different  sections  of  the  road,  No.  1 
relay  to  No.  1  section,  No.  2  relay  to  No.  2  section,  and  so  on.  The 
armatures  of  the  relays  are  opposed  by  springs.  Mr.  Willis  occasion- 
ally uses  weights  of  gradually  decreasing  tension  or  leverage.  The 
tension  of  the  spring,  or  the  leverage  of  the  weight  opposing  the  pull 
of  the  electro-magnet  of  No.  1  relay  for  its  armature  is  stronger  than 


ELECTRIC   MINING   SIGNALS   AND   TELEPHONES    27 

that  of  No.  2  ;  the  spring  of  No.  2  is  stronger  than  that  of  No.  3,  and 
so  on.  The  currents  arriving  from  the  different  sections  are  weaker 
as  the  section  is  further  from  the  engine-house,  the  current  from 
No.  1  section  being  stronger  than  that  from  No.  2  ;  the  current  from 
No.  2  stronger  than  that  from  No.  3,  and  so  on  Hence  it  follows, 
if  everything  is  in  order,  that  the  relay  of  No.  1  will  only  respond  to 
the  stronger  current  sent  from  No.  1  section;  the  relay  of  No.  2 
will  respond  to  the  current  sent  from  No.  2  section;  but  not  that 
from  Nos.  3,  4,  and  so  on.  The  current  from  any  section  passes 
through  its  own  relay,  pulls  up  its  armature,  cutting  off  the  weaker 
relays  behind  it,  closes  its  own  local  circuit  with  the  battery  pro- 
vided for  it,  and  shows,  either  by  means  of  a  disc  in  front  of  a 
glass,  or  by  a  lamp  behind  a  glass,  the  section  which  has  signalled. 
The  bell,  which  is  common  to  all  the  sections,  has  its  own  relay, 
which  is  actuated  by  the  smallest  current  and  rings  from  any 
section.  The  resistances  which  are  interposed  between  the  sections 
of  the  iron  wires  are  made  to  do  duty  as  relays  to  work  the 
return  signal  bells.  On  the  endless  rope-engine  road  it  is  usual  to 
have  a  bell  at  each  hooking-on  place,  and  in  this  case  the  engine- 
man  will  ring  all  the  bells  from  a  key  in  the  engine-house  when  he 
is  going  to  start,  so  that  all  the  hookers- on  will  know.  The  principal 
difficulty  in  connection  with  this  apparatus  is,  maintaining  the 
tensions  of  the  springs  at  their  proper  figure.  Some  years  ago,  when 
electric  signals  were  first  being  introduced  into  coal  mines,  this 
would  have  been  serious.  At  the  present  time,  when  electricity 
is  so  largely  employed  about  coal  and  metalliferous  mines,  it  ought 
to  present  no  difficulty  whatever.  The  electrician  who  can  look  after 
a  dynamo,  a  motor,  starting-gear,  etc.,  can,  if  he  will,  easily  look  after 
an  apparatus  of  this  kind,  and  it  appears  to  the  author  that  the 
application  of  the  apparatus  is  capable  of  very  considerable 
extension. 

Sources  of  Current  for  Mining  Signals 

As  is  explained  on  p.  28,  open-type  Leclanche,  open-type 
mercury  bichromate,  and  dry  batteries  are  usually  employed  for 
electric  signalling,  the  last  gradually  displacing  the  others.  In  the 
early  days  of  electric  light  in  mines,  however,  several  signals  were 
worked  by  means  of  current  taken  from  the  lighting  service ;  but 
this  is  very  dangerous,  and  is  absolutely  forbidden  by  the  Home 
Office  regulations  for  the  use  of  electricity  in  coal  mines.  There  is 
no  reason,  however,  that  in  those  mines  where  there  is  a  power 
service,  no  matter  what  it  may  be,  whether  500  volt  continuous, 
440,  500,  or  3000  three-phase  alternating,  that  it,  or  the  lighting 
service,  should  not  be  used  to  work  the  signals,  by  the  aid  of  motor 


28  ELECTRICITY  IN   MINING 

generators.  As  will  be  explained  in  Chapter  IV.,  the  motor  generator 
consists  of  two  distinct  machines,  the  axles  of  the  revolving  portions 
of  the  two  being  mechanically  connected.  One  machine  receives 
current  from  the;  supply  service,  transformed  down  if  necessary  by 
stationary  transformers,  as  will  be  explained,  to  any  convenient 
pressure.  It  runs  as  a  motor,  and  drives  the  other  machine  as 
a  generator.  The  second  machine  generates  current  at  whatever 
pressure  may  be  desired.  In  the  present  instance  it  might  be 
a  pressure  of  10  or  20  volts,  and  the  current  might  be  any  that 
was  convenient.  If  the  motor  generator  is  properly  constructed, 
there  should  be  no  connection  whatever  between  the  supply  service 
and  the  electric  signal  service,  while  the  latter  would  be  worked 
very  much  more  conveniently,  and  indicating  lamps  might  be 
added  to  the  present  bell  signals  with  great  advantage.  One  of 
the  difficulties  in  connection  with  Mr.  Willis'  indicating  signal, 
and  for  which  he  has  felt  obliged  to  provide  an  adjustable  rheostat, 
viz.  the  difference  in  the  current  strength  delivered  to  the  signal, 
owing  to  the  different  condition  of  the  battery  at  different  times 
as  its  life  increases,  would  be  completely  overcome.  With  reason- 
able attention  the  current  delivered  to  the  signals  should  be 
practically  the  same  at  all  times,  and  Mr.  Willis'  or  any  similar 
apparatus  should  easily  be  able  to  be  worked.  Further,  motor 
generators  could  be  placed  in  any  convenient  position  where  it 
would  be  handy  to  have  an  electric  signal,  the  attention  they 
require  being  very  small,  if  properly  arranged.  A  battery  of 
accumulators  may  also  be  used  for  working  any  individual  signal,  or 
for  working  all  the  signals  about  the  mine.  Mr.  Willis  informs  the 
author  that  he  has  adopted  the  latter  method  at  Leycett.  A  similar 
method,  but  with  large  Leclanche  open-type  cells,  was  adopted  at 
Annesley  Colliery  many  years  since. 

Forms  of  Galvanic  Battery 

At  the  present  time  there  are  only  three  forms  of  primary 
galvanic  battery  that  are  of  any  use  for  mining  work,  and  two  of 
them  are  giving  way  rapidly  to  the  third. 

The  forms  are  the  open-type  or  wet  Leclanche,  the  mercury 
bichromate,  and  the  dry  cell.  The  Leclanche  battery  hardly  re- 
quires description.  It  consists  of  an  outer  glass  or  earthenware  jar, 
containing  a  solution  of  sal  ammoniac,  in  which  are  immersed  a  zinc 
rod,  having  a  copper  wire  attached  to  it  for  the  purpose  of  connecting 
to  the  next  cell,  or  to  the  service,  and  a  cylindrical  porous  cell 
holding  a  lead-capped  carbon  plate  standing  in  a  mass  of  carbon  and 
oxide  of  manganese,  crushed  to  about  the  size  of  a  pea,  the  porous 
cell  being  sealed  over  with  pitch,  and  the  lead  cap  having  a  brass 


ELECTRIC   MINING  SIGNALS   AND   TELEPHONES     29 

terminal  screw  for  action.  There  are  several  chemical  actions  going 
on  in  the  Leelanche  cell  when  it  is  furnishing  current,  the  principal 
of  them  being,  the  zinc  is  dissolved  in  the  solution,  and  the  oxide  of 
manganese  is  gradually  deprived  of  a  portion  of  its  oxygen.  In 
addition,  the  sal  ammoniac  is  gradually  used  up,  and  the  pores  of  the 
cylindrical  jar  are  also  gradually  filled  up,  so  that  a  large  increased 
resistance  is  offered  to  the  passage  of  the  current. 

In  the  mercury  bichromate  cell  there  is  the  same  outer  contain- 
ing jar,  usually  of  glazed  earthenware,  holding  a  solution  of  either 
bichromate  of  potash  and  sulphuric  acid,  or  of  the  commercial 
chromic  acid,  which  contains  a  large  percentage  of  sulphuric  acid ; 
a  cylindrical  porous  cell  standing  in  the  solution,  and  having  inside 
it  a  zinc,  made  in  the  form  of  a  truncated  cone  with  a  stout  cylinder 
above,  into  which  a  copper  wire  is  cast.  The  zinc  stands  in  a  bath 
of  mercury  in  the  porous  cell,  the  latter  being  filled  at  first  either 
with  plain  water,  or  water  to  which  a  small  quantity  of  sulphuric 
acid  has  been  added.  There  are  also  many  complicated  actions 
taking  place  in  the  mercury  bichromate  cell,  the  principal  of  which 
are  the  gradual  dissolving  of  the  zinc  in  the  solution  in  which  it 
stands,  and  the  gradual  using  up  of  the  bichromate  of  potash,  or  the 
chromic  acid,  the  porous  cell  also  having  its  pores  gradually  filled 
up  as  in  the  Leelanche.  The  mercury  bichromate  cell  has  much 
better  staying  power  than  the  Leelanche,  but  it  is  sometimes  more 
difficult  to  look  after,  and  the  acid  is  more  unpleasant  to  handle  than 
sal  ammoniac. 

The  dry  cell  is  really  an  enclosed  Leelanche  cell.  It  consists, 
in  nearly  every  form,  of  a  thin  hollow  cylinder  of  zinc,  and  a  carbon 
rod  with  a  mass  of  crushed  carbon  and  manganese  compressed  round 
it,  the  rod  with  its  manganese  standing  in  the  middle  of  the  zinc 
cylinder,  and  the  space  between  it  and  the  zinc  being  filled  with  a 
pasty  mass,  consisting  either  of  plaster  of  Paris,  gypsum,  or  other 
substance,  mixed  with  a  solution  of  sal  ammoniac.  The  top  of  the 
zinc  cylinder  is  sealed  over  with  pitch,  and  in  the  best  forms  the 
whole  is  placed  inside  a  glazed  porcelain  cylinder.  The  action  of 
the  dry  cell  is  exactly  the  same  as  that  of  the  open  Leelanche,  and 
its  useful  life,  its  ability  to  do  work,  depends  upon  the  quantity  of 
oxide  of  manganese  that  can  be  usefully  exposed  to  the  current,  and 
upon  the  quantity  of  the  solution  of  sal  ammoniac  that  can  be  held  in 
the  pasty  mass  of  plaster  of  Paris,  etc.  The  dry  cell  is  making 
headway  because  it  is  so  clean  and  so  convenient.  The  battery  man 
can  easily  take  a  few  dry  cells  down  the  pit  in  his  bag,  or  can  keep 
a  few  in  a  cool  place  near  the  battery,  and  it  is  a  very  simple  matter 
to  change  any  cells  that  are  run  down.  They  are  not  so  economical 
in  material  as  either  the  open-type  Leelanche  or  the  mercury 
bichromate,  but  in  the  great  majority  of  cases  they  are  very  much 


ELECTRICITY   IN   MINING 


more  economical  in  attendance,  and  the  labour  saved  more  than  pays 
for  the  additional  material  used.  Unfortunately  there  are  only  a 
few  forms  of  dry  cell  that  are  really  reliable,  though  improvements 
in  manufacture  are  gradually  bringing  more  and  more  into  the 
market. 

Fitting  up  Engine-road  Signals. —  Engine-road    signals   are 
usually  fitted  with  No.  8  galvanized  iron  wire,  stretched  tightly  along 

one  side  of  the  road,  and  fixed 
to  small  reel  insulators  of  glazed 
earthenware.  The  insulators  are 
about  1J  inches  in  diameter. 
They  have  a  hole  through  the 
centre  large  enough  to  take  a 


FIG.  9.— Beel  Insulators  used  with 
Engine-road  Signals. 


No.  20  iron-wood  screw,  and 
they  have  a  groove  on  the  edge 
in  which  the  galvanized  iron 
lays.  The  wire  is  bound  to  the  insulator  by  thin  galvanized  iron 
wire.  They  are  shown  in  Fig.  9. 

Note. — In  binding  wires  to  insulators,  whether  the  wires  are 
covered  or  not,  be  sure  always  to  use  the  same  kind  of  wire  for  the 
binding  as  the  conductor  is  made  of.  Use  galvanized  wire  for 
binding  in  galvanized  iron  or  steel,  and  copper  wire  for  binding 
copper  conductors,  never  the  reverse  of  these,  or  galvanic  action  will 
be  set  up  that  will  inevitably  lead  to  trouble. 

Another  form  of  insulator  that  has  been  employed  is  one — a 
modification  of  that  which  is  used  for  fixing  covered  electric  light 
wires  in  buildings — where  the  wires  are  not  laid  in  wood  boxing  or 
conduits.  The  insulator  is  made  in  two  halves,  that  is  to  say,  two 
discs,  each  having  a  hole  for  the  screw  in  the  centre,  and  each  having 
a  groove  cut  along  the  face  of  the  parts  of  the  insulator  which  come 
together,  in  which  the  wire  can  lie  and  be  held  tight  by  screwing 
the  two  parts  of  the  insulator  together.  Another  form  of  insulator 
that  has  been  employed  is  a  reel  made  of  vulcanized  indiarubber. 
It  is  rather  less  expensive  than  the  glazed  earthenware,  does  not 
break  so  easily,  but  is  not  as  strong.  It  is  shown  in  Fig.  10.  For 
straining  iron  wires,  too,  at  the  ends  of  the  roads,  a  very  much 
stronger  insulator  is  employed,  known  as  the  shackle  or  shackle 
insulator.  It  is  3  inches  in  diameter,  about  2  inches  thick,  with  a 
hole  in  its  centre  large  enough  to  take  a  |-inch  bolt,  and  a  groove 
about  its  edge  large  enough  for  two  thicknesses  of  the  iron  wire 
to  lie  in.  It  is  shown  in  Fig.  11.  The  shackle  insulator  is  held 
between  two  galvanized  iron  straps  by  means  of  a  galvanized  iron 
bolt  or  nut,  the  other  ends  of  the  straps  being  secured  to  a  prop 
or  any  convenient  position  by  an  iron-wood  screw.  The  end  of  the 
iron  wire  is  taken  twice  round  the  groove  in  the  body  of  the 


ELECTRIC   MINING   SIGNALS   AND   TELEPHONES    31 

insulator,  and  is  secured  by  being  twisted  several  times  round  its 
own  part.  The  iron  wire  is  then  stretched  out  to  the  full  length 
of  the  road  by  means  of  a  telegraph  wire-man's  straining  vice. 
The  straining  vice  consists  of  a  hand-vice  attached  to  a  small  drum 


FIG.  10. — Forms  of  Vulcanized  Rubber  Insulators  made  by  the 
Avon  Rubber  Co. 


with  a  ratchet  and  paul,  the  connection  between  the  two  being 
by  means  of  a  swivel.  The  vice  is  clipped  on  the  wire,  a  small 
length  of  another  wire  is  attached  to  the  small  drum,  and  the  wire 
is  tightened  up  to  a  prop  ahead,  another  vice  tightening  up  in  front 


FIG.  11. — Forms  of  Shackle  Insulators  used  for  terminating 
Engine-road  Signal  Wires. 

to  another  prop,  the  whole  being  secured  to  the  insulators  after 
tightening.  The  wires  should  be  "  shackled  off,"  as  it  is  termed,  on 
each  side  of  each  junction  of  the  engine  road — say  at  each  hitching- 
on  place— the  connection  between  the  different  sections  being  made 


32  ELECTRICITY  IN    MINING 

by  means  of  insulated  wire.  It  will  be  seen  that  it  is  easy  to  insert 
Mr.  Willis'  relays  at  the  different  junctions  by  this  method.  In  the 
engine-house  the  usual  circuit  is  formed  of  a  battery,  now  nearly 
always  consisting  of  dry  cells ;  a  bell,  which  may  be  single  stroke 
or  trembling,  but  is  more  frequently  the  latter ;  and  a  switch,  which 
may  be  of  the  plug  or  lever  form,  to  disconnect  the  battery  when  the 
signal  is  not  required  to  be  used.  The  author  would  very  strongly 
advise  the  addition  to  the  above  of  a  low-reading  ampere  meter.  It 
should  be  made  to  read  in  one-hundredth  parts  of  an  ampere.  Instru- 
ments are  now  made  for  all  ranges.  Milliampere  meters  are  quite 
common,  and  there  should  be  no  difficulty  in  making  centiampere 
meters.  The  reason  the  author  so  strongly  recommends  this  is,  the 


ENQINC 
BELL 


fl 


u 


FIG.  12. — Diagram  of  Connections  of  a  Shaft  Signal  with  three  Bells,  two 
Batteries,  and  two  Ringing-keys. 

great  difficulty  with  engine-road  signals  is  leakage  of  the  current. 
Where  roads  are  wet,  and  where  especially  insulators  are  allowed  to 
be  broken  and  not  replaced,  insulation  of  the  wires  steadily  goes 
down,  with  the  result  that  faulty  signals  are  given  after  a  time. 
The  presence  of  the  centiampere  meter  would  show  the  electrician  in 
charge,  the  gradual  increase  of  his  leakage  current,  and  it  would  also 
be  an  additional  guide  to  the  engine-man  when  a  signal  was  given. 
When  there  is  a  large  leakage,  the  single-stroke  bell  is  apt  to  keep 
its  armature  up  to  its  magnet,  while  the  trembler  bell  is  apt  to 
maintain  a  continuous  chattering,  the  result  being  in  either  case  that 
the  engine-man  has  to  judge  by  his  experience  when  a  signal  has 


ELECTRIC   MINING   SIGNALS   AND  TELEPHONES     33 


been  given.  With  a  centiampere  meter  in  the  circuit,  the  throw  of 
the  needle  when  a  signal  was  given  would  be  a  very  useful  additional 
indication. 

Very  Important  Note.  —  The  wires  for  engine-road  signals 
should  be  kept,  under  all  circumstances,  well  away  from  cables 
carrying  currents  for  the  electric  light  or  power  service,  no  matter 
whether  the  latter  are  armoured  or  unarmoured.  Where  it  is 
possible,  signal  wires  should  be  on  the  opposite  side  of  the  road  to 
the  cables,  and,  in  any  case,  should  be  so  fixed  that  falls  of  roof,  or 
falls  of  the  wires  or  cables  themselves,  should  not  bring  the  two  into 
contact.  In  the  author's  opinion,  some  of  the  accidents  that  have 
taken  place  where  men  have  received  shocks  from  touching  signal 
wires  are  due  to  a  want  of  this  precaution. 

Shaft  Signals 

Shaft  signals  are  used  for  signalling  from  the  bottom  to  the  bank 
and  to  the  engine-house,  from  the  engine-house  and  the  bank  to  the 
bottom,  and  from  the  bank 
to  the  engine-house.  The 
usual  arrangement  is,  a 
powerful  single  stroke  bell 
is  fixed  at  each  pit  bottom, 
one  on  the  bank  for  each 
pit  bottom,  and  one  in  the 
engine-house  for  each  pit 
bottom.  Covered  copper 
wires,  which  are  sometimes 
as  small  as  No.  18,  but  which 
should  never  be  less  than 
No.  16,  and  would  be  better 
if  No.  14,  connect  the  bells 
with  the  batteries,  and  with 
the  arrangements  for  com- 
pleting the  circuit.  There 
may  be  one  or  two  batteries. 
The  author  prefers  two,  one 
to  furnish  current  for  the 
up  signals,  this  battery  being 
fixed  at  each  pit  bottom,  and 
the  other  to  furnish  current 
for  the  down  signals  and 
those  between  the  bank  and 
engine-house,  this  being  fixed  on  the  surface.  His  reason  is  that  the 
covered  wires  in  the  shaft  are  the  most  difficult  to  maintain,  and  the 

P 


Fia.     13. — Iron-cased    Einging-key    for    Shaft 
Signals  made  by  the  Electric  Ordnance  Co. 


34 


ELECTRICITY  IN   MINING 


wire  carrying  the  current  from  the  surface  to  the  pit  bottom  is  always 
very  much  more  difficult  to  maintain,  very  much  more  liable  to  elec- 
trolytic action,  than  those  in  which  the  current  only  passes  when  a 
signal  is  given.  The  connections  for  this  are  shown  in  Fig.  12. 
The  ringing  apparatus,  or  "pushers,"  as  mining  men  term  them,  are 


PIG.  14. — Ringing-key,  shown  in  Pig.  13,  arranged  to  be  worked 
by  a  Lever  and  Rope  instead  of  by  the  Band. 


now  nearly  always  of  the  plunger  form.  A  lever  or  spring  is 
enclosed  inside  an  iron  dome-shaped  box,  fitted  so  as  to  be  perfectly 
water-tight,  and  from  the  top  of  the  dome  a  short  cylinder  projects, 
carrying  a  plunger,  ending  in  a  substantial  wooden  or  vulcanized 
knob,  the  plunger  being  kept  out  of  contact  with  the  lever  or  spring 

by  a  stout  spiral  spring  surround- 
ing it.  The  circuit  is  completed 
by  pushing  the  plunger  inwards, 
and  bringing  the  spring  with  its 
contact  piece,  or  the  lever,  into 
connection  with  the  contact  piece 
provided  for  it.  A  form  of  ring- 
ing-key is  shown  in  Fig.  13,  to 
be  worked  by  hand,  in  Fig.  14 
to  be  worked  by  a  lever,  and  a 
section  is  shown  in  Fig.  15.  Even 


FIG.  15. — Section  of  the  Ringing-key 
shown  in  Figs.  13  and  14.  B  is  a 
Triple  Wire;  C  a  Gland  through 
which  it  passes ;  E  the  Case. 


with  the  very  best  fitting,  there 
is  considerable  difficulty  in  pre- 
venting water,  which  is  nearly 
always  present  at  pit  bottoms, 
in  the  positions  where  the  ringing-keys  have  to  be  fixed,  from 
creeping  into  the  contact-box.  The  author  prefers  to  have  the 
terminals  to  which  the  wires  leading  to  the  battery  and  the  bell  are 
to  be  connected  outside  of  the  contact- box,  so  that  any  electrolytic 


ELECTRIC   MINING   SIGNALS   AND   TELEPHONES     35 

or  electro- chemical  action  may  be  seen,  and  the  oxide  and  copper 
which  is  formed  cleaned  off  periodically. 

If  the  author's  suggestion  is  adopted,  and  motor  generators  are 
employed  to  furnish  current  for  the  signals,  the  current  for  the  up 
signal  can  be  taken  from  a  motor-generator  at  the  pit  bottom,  and 
that  for  the  down  signals  from  one  on  the  bank,  or  the  same  arrange- 
ment can  be  made  with  accumulators. 


Bells  for  Mining  Signals 

Bells  for  mining  signals  are  of  two  forms,  those  that  are  intended 
to  be  used  in  explosive  atmospheres,  and  those  that  are  to  be  used  in 
ordinary  atmosphere.  The  author  has  made  a  good  many  experiments 
to  determine  whether  it  is  possible  for  the  spark  which  passes  between 
the  contacts  in  a  trembler-bell  to  ignite  an  explosive  mixture,  and  the 

conclusion  he  came  to  was  that  it  was 
not.  He  understands,  however,  that 
others  have  succeeded  in  igniting  an 
explosive  mixture  by  means  of  the 


FIG.  16. — Gas-proof  Iron-cased 
Trembler-bell,  for  Mining 
Signals,  made  by  the  Electric 
Ordnance  Co. 


FIG.  17. — Single- stroke  Iron-cased  Damp 
and  Gas-proof  Mining  Bell,  made  by 
the  Electric  Ordnance  Co. 


spark  mentioned,  and  as  one  positive  experiment  of  this  kind  is 
of  more  importance  than  a  thousand  negative  ones,  he  proposes  to 
describe  what  has  been  done  to  meet  the  case.  Briefly,  what  has 
been  done  is  to  enclose  the  trembling  contact  of  the  trembler-bell 
inside  a  gas-proof  case,  and  to  cause  the  armature  of  the  bell  to 


36  ELECTRICITY   IN   MINING 

transmit  the  trembling  action  to  the  hammer-rod  which  strikes  the 
bell,  by  means  of  a  mechanical  arrangement,  acting  through  a  metallic 
diaphragm.  One  of  these  arrangements  is  shown  in  Fig.  16.  The 
construction  of  bells  for  mining  work  should  be  very  strong.  The 
electro-magnets  should  be  very  powerful,  and  the  whole  thing  should 
be  so  arranged  that  the  bell  will  go  on  working,  though  the  battery 
has  run  down  to  a  certain  extent.  Further,  bells  which  are  intended 
for  mining  work,  and  particularly  those  for  metalliferous  mines,  should 
have  their  electro-magnetic  apparatus  enclosed  inside  of  damp-proof 
cases,  the  motion  of  the  hammer-shaft  being  delivered  to  it  in  such  a 
manner  that  the  damp-proof  arrangement  will  not  be  broken.  Fig.  17 
shows  a  single-stroke  bell,  embodying  this. 

Relays  for  Mining  Signals 

The  relay  is  a  well-known  device  used  by  telegraph  and  telephone 
engineers,  and  its  office  is  to  enable  a  very  weak  current  arriving,  say, 


BEILL 


FIG.  18. — Diagram  of  the  Connections  of  a  Two-wire  Engine-road  Signal,  with 
Belay,  two  Batteries,  Switch,  and  Centiammeter. 

from  a  distant  station  over  a  badly  insulated  line,  to  actuate  a  battery 
of  sufficient  power  to  operate  an  apparatus  that  will  give  strong,  clear 


ELECTRIC   MINING  SIGNALS   AND   TELEPHONES     37 


signals.  The  device  is  employed  in  some  forms  of  wireless  telegraphy, 
the  electric  waves  causing  a  circuit  in  which  a  very  weak  current  is 
passing  to  be  completed,  and  to  operate  a  relay  which  actuates 
a  circuit  containing  a  powerful  battery  and  printing  apparatus. 
The  relay  was  introduced  into 
mining  signal  work  by  the  author 
some  twenty-five  years  ago,  to 
meet  a  case  where  he  was  obliged 
to  use  very  weak  currents,  owing 
to  the  fact  that  the  long  engine 
road  upon  which  the  signals  were 
working  was  very  wet,  and  it  was 
impossible  to  maintain  the  insula- 
tion. The  only  method  of  keeping 
the  signals  going  was  by  using  a 
very  weak  battery,  and  causing  it 
to  actuate  a  relay  which  brought 
a  powerful  battery  into  operation, 
completing  a  circuit  in  which  the 
bell  to  be  worked  was  included, 
the  latter  giving  a  loud,  clear  sound. 
The  connections  for  this  are  shown 

in  Fig.  18.  The  relay  has  since  been  employed  for  other  similar  work, 
as,  for  instance,  for  ringing  each  bell  of  a  number  of  bells  on  an 
endless  rope  engine  road  by  means  of  its  own  battery,  fixed  locally ; 
and,  as  explained,  it  has  been  made  the  base  of  Mr.  Willis's  ingenious 
apparatus  for  indicating  signals  on  endless  rope  engine  roads.  The 
remarks  made  in  connection  with  electric  bells  for  mining  signalling 
generally  apply  to  relays.  The  relay  is  a  very  much  smaller  apparatus 
than  either  the  signal  stroke  or  trembler-bell,  but  its  electro-magnet 
should  be  made  fairly  powerful  for  the  work  it  is  intended  to  do,  and 
it  should  be  enclosed  in  a  damp-proof  and  gas-proof  case.  One  form 
is  shown  in  Fig.  19. 


FIG.  19. — Relay  enclosed  in  Iron  Case, 
for  Mining  Work. 


Telephones  for  Mines 

For  communicating  between  different  mines  and  works  under  the 
same  management,  and  between  the  different  parts  of  the  same  works, 
perhaps  the  simplest  method,  when  the  establishment  is  large  enough, 
is  that  of  the  telephone  exchange.  At  each  mine  wires  are  brought 
from  each  point  between  which  it  is  required  to  communicate,  to 
sub-centres,  wires  are  taken  from  the  sub- centres  to  centres,  and 
wires  again  connect  the  centres  at  the  different  mines.  At  each 
sub- centre  there  is  a  simple  telephone  switchboard,  consisting  of  a 


38  ELECTRICITY   IN   MINING 

number  of  electro-magnets,  each  having  its  own  shutter,  which  drops 
and  discloses  its  number  when  a  call  is  made,  and  a  system  of 
connecting  arrangements,  either  by  plugs  arranged  to  connect  any 
one  of  a  number  of  vertical  bars  to  any  one  of  a  number  of  horizontal 
bars,  or  by  what  are  termed  "jacks,"  consisting  of  vulcanite  plugs 
with  connecting  pieces,  the  plugs  being  forced  into  certain  holes 
under  the  indicating  electro-magnets,  making  connection  between 
the  ends  of  the  coils  of  the  electro-magnets  and  wires  concealed 
in  a  cord  attached  to  the  "jack."  At  the  other  end  of  the  cord 
there  is  another  "  jack,"  and  when  connection  is  required  between 
any  two  numbers  of  the  sub-centre,  one  "  jack "  is  inserted 
under  the  indicator  of  the  calling  number,  and  the  other  "jack"  is 
pushed  in  under  the  indicator  of  the  number  wanted.  The  wire 
leading  to  the  centre  has  its  own  indicator,  similar  to  those  of  the 
numbers  of  the  sub-centre,  and  when  connection  is  required  with  a 
number  in  another  sub-centre,  the  centre  is  called,  connection  is  made 
with  it,  the  centre  calls  the  sub-centre  where  the  number  wanted  is, 
the  second  sub-centre  calls  its  number,  makes  connection  with  it, 
signals  the  centre,  which  signals  the  first  sub-centre,  and  connection 
is  complete.  Arrangements  are  made  by  ringing  a  bell,  or  in  other 
ways,  to  advise  the  sub-centre  and  the  centre  when  communication  is 
finished.  This  arrangement  can  be  carried  almost  as  far  as  the 
management  choose.  The  only  point  in  connection  with  it  that  has 
to  be  considered  is,  each  switchboard  requires  attention,  but  as  one 
sub-centre  might  be  in  the  weigh  cabin  at  the  pit  top,  the  weigher 
might  easily  attend  to  it,  and  one  of  the  clerks  in  the  central  office 
could  easily  attend  to  the  centre  switchboard.  Automatic  exchanges 
are  on  the  market,  and  are  said  to  be  doing  good  work  in  America ; 
but,  so  far,  they  do  not  appear  to  have  made  any  headway  in  this 
country,  and  the  author  fears  that  the  apparatus  would  be  too  delicate 
for  use  about  coal  mines.  In  the  automatic  apparatus  each  telephone 
station  has  its  number,  and  the  user  signals  his  number  to  the  sub- 
centre  or  the  centre  by  turning  a  lever  to  certain  numbers  in 
succession,  and  a  succession  of  electro-magnets  perform  the  operation 
of  making  successive  connections,  just  as  the  attendants  of  a  telephone 
exchange  would. 

Telephone  exchanges  about  mines  are  conveniently  worked  by 
batteries  of  dry  cells,  but  they  also  might  be  worked  by  means  of 
motor  generators,  giving  low  pressure  currents,  or  from  accumulators. 

Intercommunication  Telephone  Apparatus.  —  Where  the 
number  of  telephone  stations  is  not  large,  and  there  is  no  convenience 
for  sub- centres,  the  intercommunication  telephonic  system  is  very 
convenient.  By  this  arrangement  each  station  has  its  own  telephone 
set,  consisting  of  transmitter,  receiver,  switches,  etc.,  and  in  addition, 
it  has  a  multiple  switch,  the  number  of  contacts  on  which  are  as 


ELECTRIC   MINING   SIGNALS   AND   TELEPHONES     39 

many  as  the  number  of  stations  that  can  be  connected  on  the  system. 
Each  station  has  its  own  number,  and  communicates  with  any  other 
station  by  turning  a  switch  handle  to  the  number  of  the  station  it 
wishes  to  speak  to,  calling  and  talking  then  in  the  usual  way.  Some 
trouble  arises  in  intercommunication  sets  if  the  switch  handle  is  not 
returned  to  0,  which  is  the  receiving  number  for  all  the  stations. 
When  the  station  is  not  receiving  a  message,  the  switch  handle 
should  be  in  the  position  for  calling.  With  some  systems  the 
difficulty  has  been  overcome  by  the  switch  handle  being  automati- 
cally brought  back  to  the  calling  position  when  the  telephone  receiver 
is  replaced  on  its  hook.  In  other  cases,  as  with  the  Ericsson  and  the 
Berliner,  special  arrangements  are  made,  so  that,  notwithstanding  a 
switch  may  be  left,  say,  on  No.  6  at  No.  10  station,  if  any  other  station 
calls  No.  10,  the  call  will  reach  No.  10's  bell,  and  communication 
can  be  entered  into.  Another  difficulty  which  arises  in  some  cases 
in  connection  with  the  intercommunication  system  is,  if  one  wire 
is  used  as  a  common  return,  all  of  the  stations  can,  if  they  choose, 
hear  any  message  that  is  being  sent  between  any  two  stations.  This 
has  been  overcome  in  the  case  of  the  two  firms  mentioned,  by  using 
complete  metallic  circuits,  and  by  doubling  the  switching  arrange- 
ment. With  one  common  return  wire  there  must  be  the  same 
number  of  wires  leading  to  all  the  stations  as  there  are  stations,  plus 
two,  the  wires  for  the  battery  and  return.  That  is  to  say,  if  there  are 
twenty  stations,  there  must  be  twenty-two  wires  leading  into  each 
station.  This  was  a  little  formidable  at  first,  but  the  difficulty  has 
been  overcome  by  making  up  cables  with  the  required  number  of 
wires,  each  properly  insulated,  and  the  whole  being  inside  an  outer 
braid,  the  complete  cable  not  being  larger  than  an  electric  light 
cable,  for  a  comparatively  small  number  of  lamps.  For  junctions 
with  the  intercommunication  system,  junction  boxes  have  been 
devised  which  simplify  the  matter  very  much.  The  wires  enter  the 
boxes,  and  are  connected  to  terminals  from  which  the  wires  leading 
to  the  station  are  also  taken,  or  from  smaller  terminals  connected 
to  the  first  terminals,  the  whole  being  enclosed  in  a  wood  or 
iron  case. 

Telephonic  Communication  on  Engine  Roads 

This  is  perhaps  the  most  useful  application  of  the  telephone  about 
a  mine,  and  it  is  applied  in  a  very  simple  manner.  The  usual 
arrangement  is,  an  ordinary  telephone  transmitter  and  receiver  is 
fixed  in  the  engine-house,  and  is  connected  to  the  engine-road  signal, 
a  switch  being  provided  to  reduce  the  battery  power  passing  in  the 
microphone  when  the  telephone  is  in  use,  this  being  necessary,  and 
the  regular  telephone  switch  keeping  the  apparatus  disconnected 


4o 


ELECTRICITY   IN   MINING 


from  the  engine-road  wires  when  not  in  use.  The  equipment  for  the 
engine  road  is  merely  one  or  two  watch  telephones  with  their  cords, 
and  clips,  to  enable  them  to  be  hooked  on  the  signal  wires.  One 
difficulty  in  connection  with  this  arrangement  is,  the  man  who  uses 
the  telephone  on  the  engine  road  has  no  means  of  knowing  that  he 

has  been  heard,  except 
by  keeping  a  telephone 
to  his  ear,  and  listening 
to  what  is  taking  place 
at  the  instrument  in 
the  engine-house.  If 
the  engine-man  is  busy, 
and  he  cannot  attend  to 
the  call,  which  is  made 
by  rapping  a  certain 
number  of  times  on  the 
signal-bell,  it  becomes 
very  tedious  and  tiring 
for  the  road-man  to  hold 
the  telephone  at  his  ear. 
The  author  suggests 
that  a  simple  apparatus, 
consisting  of  a  couple 
of  small  dry  cells,  a 
small  bell,  switches, 
etc.,  t  made  up  in  a 
convenient  form,  and 
carried  in  a  leather 
case,  might  be  used  by 

FIG.  20.— Simple  Telephonic  Apparatus  for  Use  in  the  road  men,  and  when 

Mines,  consisting  of  Microphone  Transmitter,  they    wished     to    COm- 
Telephone   Receiver,   and   Switch,  the   Tele-  J  -     ,        ,  T 

phone  Cord  and  the  Receiver  being  protected  mumcate     they    might 

by  a  wrapping  of  Brass  Tubes.  signal    the    number    of 

raps,  and   in   place  of 

keeping  the  telephone  to  their  ear,  hook  their  apparatus  and  the 
wires  and  wait  till  the  engine-man  signalled.  The  apparatus  would 
not  be  expensive,  and  it  would  be  very  convenient.  A  handy 
form  of  telephonic  apparatus  for  use  in  mines  is  shown  in  Fig.  16. 

Wireless  Telegraphy  for  Mines 

Wireless  telegraphy  cannot  be  used  for  communicating  with  the 
underground  workings,  and  it  is  very  doubtful  whether  much 
advantage  would  be  gained  by  its  use  for  communicating  between 
mines  situated  at  a  distance,  though  the  author  has  been  informed 


ELECTRIC   MINING   SIGNALS   AND   TELEPHONES     41 

that  a  manufacturer  in  Yorkshire  communicated  with  the  works 
from  his  house  by  wireless  telegraphy.  Wireless  apparatus  is  all 
very  delicate,  and  it  writes  Morse  characters,  while  the  telephone 
can  be  used  by  any  one,  and  speaks  the  language  of  whoever  is 
using  it.  In  addition,  the  Government  have  claimed  a  monopoly 
of  its  use. 

Shot  Firing  by  Electricity 

By  shot  firing  is  meant  the  ignition  of  the  charges  of  explosives 
employed  to  force  the  mineral  down  clear  of  the  other  strata,  as  will 
be  explained  in  connection  with  coal  cutting  and  drilling  in 
Chapter  VI.  Every  explosive,  and  in  fact  every  substance,  has 
a  certain  ignition  temperature,  and  to  ignite  the  explosive  a  small 
portion  of  it  has  to  be  raised  to  the  ignition  temperature,  the  heat 
generated  in  the  burning  of  this  portion  being  sufficient  to  ignite  the 
rest,  rapid  combustion  and  what  we  call  explosion,  very  rapid  expan- 
sion of  the  gases  formed  by  the  combustion  of  the  explosive,  following. 
With  electric  ignition  the  required  increase  of  temperature  is  pro- 
duced by  the  ignition  of  a  small  quantity  of  a  specially  sensitive 
substance,  fulminate  of  mercury  being  the  one  most  commonly 
employed,  which  is  itself  raised  to  its  ignition  temperature  by  the 
heat  liberated  either  in  a  small  platinum  wire,  or  by  a  spark. 
Electric  fuses,  as  they  are  called,  consist  of  small  quantities  of  one 
of  the  substances  mentioned,  in  which  is  embedded  either  a  small 
platinum  wire,  as  described,  or  the  ends  of  two  copper  wires.  In 
either  case  the  fuse  and  wires  are  enclosed  inside  a  copper  cap,  for 
protection,  and  the  ends  of  the  wires  are  brought  down  to  insulated 
connecting  wires,  which  are  sometimes  fixed  in  grooves  in  a  stick,  to 
one  end  of  which  the  fuse  is  attached,  the  stick  being  of  the  length 
of  the  shot  hole.  The  connecting  wires  in  any  case  must  be  long 
enough  to  allow  the  fuse  to  be  pushed  right  up  against  the  end  of  the 
charge  of  explosive,  to  extend  to  the  mouth  of  the  shot  hole,  and  to 
leave  sufficient  length  for  connecting  to  the  source  of  current.  The 
fuses  in  which  the  small  platinum  wire  is  embedded,  are  known  as 
low  tension  fuses,  and  the  source  of  current  required  to  raise  them  to 
the  temperature  necessary  to  fire  the  fuse,  is  provided  from  a  source 
of  electricity  of  low  tension,  but  that  will  furnish  a  comparatively 
large  current,  the  current  being  0*3  amperes  for  each  fuse,  while  the 
pressure  need  only  be  a  few  volts.  The  current  for  low  tension  fuses 
is  generally  supplied  by  a  battery  of  either  dry  cells,  or  accumulators. 
The  latter  are  more  reliable,  providing  that  care  is  taken  before 
carrying  them  down  the  pit,  to  see  that  they  are  properly  charged. 
They  require  recharging  from  time  to  time,  and  it  must  be  remembered 
that  if  the  firing  accumulator  battery  is  not  used  for  some  time,  it 


42  ELECTRICITY   IN   MINING 

must  still  be  charged  as  often  as  the  pressure  falls  to  1/8  volts  per 
cell.     It  is  a  good  plan  to  give  them  a  charge  every  day. 

The  fuses  in  which  the  heat  is  generated  by  the  passage  of  a 
spark  are  known  as  high  tension  fuses,  and  the  pressure  required 
may  vary  considerably,  though  in  modern  fuses  the  results  are  much 
more  uniform  than  in  those  of  twenty  years  ago.  The  lengths  of  the 
wires  protruding  into  the  detonator,  their  distance  apart,  and  other 
conditions  alter  the  pressure  necessary  to  drive  a  spark  across  the  gap 
between  them.  It  may  be  taken,  however,  that  a  pressure  of  50  volts 
should  be  available,  and^it  is  all  the  better  if  the  available  pressure  is 
100  volts.  The  source  of  current  with  high  tension  fuses  is  usually  a 
magneto-generator,  a  small  dynamo  machine  fitted  with  permanent 
steel  magnets,  and  with  what  is  known  as  a  shuttle  armature ;  an 
armature  something  of  the  shuttle  form,  a  long  coil  of  wire  being 
wound  in  the  space  where  the  thread  would  be  wound  in  weaving,  the 
ends  of  the  coil  of  wire  being  connected,  usually,  one  to  the  mass  of 
the  shuttle  itself,  and  the  other  to  an  insulated  contact  on  the  end 
of  the  spindle  to  which  the  shuttle  is  attached,  and  on  which  it 
revolves,  the  contract  bearing  against  a  spring  carried  inside  the  case 
for  the  purpose.  In  some  forms  of  apparatus  a  small  condenser,  con- 
sisting of  leaves  of  tinfoil,  separated  by  paraffin  paper,  and  enclosed 
between  two  metal  plates,  is  added  to  increase  the  pressure  of  the 
firing  current.  In  any  case  the  whole  is  enclosed  inside  a  mahogany 
or  teak  box,  with  a  leather  strap  for  carrying,  and  with  a  handle 
arranged  to  fit  on  the  axle  of  the  armature,  the  latter  projecting 
through  a  hole  in  the  case  provided  for  the  purpose.  There  is  usually 
also  a  push  fixed  on  the  side  of  the  case,  convenient  to  the  hand  of 
the  shot  firer,  which  breaks  the  connection  between  two  springs, 
arranged  to  short  circuit  the  coils.  A  pair  of  terminals  for  the  wires 
to  connect  the  apparatus  to  the  fuses  completes  the  equipment.  When 
shots  are  to  be  fired,  the  charges  are  placed  in  the  shot  holes,  the  fuses 
placed  in  the  shot  holes,  and  tamping  put  in  to  fill  up  the  hole,  this 
last  being  preferably  formed  of  wet  grass  or  some  material  that  will 
extinguish  the  flame  if  the  shot  blows  outwards,  instead  of  expending 
its  energy  in  bringing  down  the  mineral.  Wires  are  then  taken  from 
the  fuses  from  the  neighbourhood  of  the  firing  battery,  using  the  term 
to  mean  the  battery  of  cells  for  the  low  tension  fuses,  or  the  small 
dynamo  machine.  The  cable  is  preferably  made  up  as  a  twin  cable, 
two  wires  separately  insulated  laid  up  together  and  insulated  overall, 
as  it  is  more  convenient  to  handle,  the  cable  being  coiled  on  a  drum 
provided  for  the  purpose,  and  placed  on  the  ground  in  the  neighbour- 
hood of  the  firing  battery.  Both  wires  of  the  cable  should  be  well 
insulated.  The  firing  battery  should  be  placed  well  out  of  the  range 
of  the  effects  of  a  possible  blown-out  shot,  and  of  all  accidents.  The 
cable  should  never  on  any  account  be  connected  to  the  firing  battery, 


ELECTRIC   MINING  SIGNALS   AND   TELEPHONES     43 

until  it  has  first  been  connected  to  the  fuses,  and  it  has  been  ascer- 
tained that  it  is  clear  of  everything  between  the  drum  and  the  fuses, 
and  that  everybody  is  out  of  the  way.  When  this  has  been  ascer- 
tained, the  inner  ends  of  the  two  wires  on  the  drum  are  connected  to 
the  terminals  of  the  firing  battery,  and  if  it  is  a  low  tension  battery, 
a  push  or  contact  key  is  pressed,  this  completing  the  circuit  between 
the  battery  and  the  fuses.  If  it  is  a  magneto-exploder,  the  handle  is 
fixed  on  the  armature,  the  shuttle  is  turned  rapidly  until  it  has  got 
up  a  good  speed,  and  the  push  is  then  pressed. 

The  fuses  may  be  either  in  series  or  in  parallel.  Series  means 
the  same  thing  as  is  shown  in  Fig.  1,  p.  7,  one  end  of  the 
connecting  wire  being  connected  to  one  wire  of  the  end  fuse,  the 
other  end  of  the  connecting  wire  to  one  wire  of  the  fuse  at  the  other 
end  of  the  row,  and  the  circuit  being  completed  by  connecting 
adjacent  wires  of  the  intermediate  fuses  together,  the  current  in  this 
case  passing  through  the  fuses  in  succession.  This  arrangement  has 
the  disadvantage  that  the  pressure  furnished  by  the  firing  battery 
must  be  sufficient  for  the  whole  number  of  fuses.  That  is  to  say,  if 
each  fuse  takes  as  much  as  50  volts,  and  there  are  six  shots  to  be 
fired,  the  firing  battery  must  furnish  300  volts.  It  also  has  the 
disadvantage  that  if  one  of  the  fuses,  as  sometimes  happens,  fires 
slightly  before  the  others,  it  breaks  the  circuit,  and  no  current  can 
pass  to  the  remainder. 

The  parallel  system  is  the  same  as  shown  in  Fig.  2,  p.  7,  each 
fuse  being  connected  between  the  two  connecting  wires,  and  the 
current  passing  simultaneously  to  all  of  them.  With  this  arrange- 
ment it  is  necessary  that  the  current  furnished  shall  be  sufficient  for 
the  whole  number  of  the  fuses.  Thus,  if  each  fuse  takes  0'3  amperes, 
the  battery  must  furnish  1/8  amperes  for  six  fuses.  It  is  also  necessary 
that  the  resistance  of  the  individual  fuses  should  be  very  nearly  alike. 
If  the  resistance  of  one  fuse  is  appreciably  lower  than  the  rest,  the 
current  passing  through  that  fuse  may  lower  the  pressure  delivered 
to  the  others  so  much  that  sufficient  current  does  not  pass  through 
them  to  ignite  them.  Per  contra,  if  one  of  the  fuses  is  of  very 
much  higher  resistance  than  the  remainder,  it  may  not  obtain 
sufficient  current. 

In  practice  it  is  usual  to  fire  high  tension  fuses  in  series,  and  low 
tension  fuses  in  parallel,  though  in  the  author's  opinion  the  reverse 
arrangement  would  be  more  satisfactory. 

Frictional  Electrical  Firing  Apparatus 

There  is  another  apparatus  that  is  still  used  occasionally  for  shot 
firing,  but  which  is  gradually  going  out,  as  the  magneto-exploder  and 
the  accumulator  and  dry  cell  become  more  perfect,  viz.  the  frictional 


44  ELECTRICITY   IN   MINING 

electrical  apparatus.  In  the  latest  form  of  this  apparatus  an  ebonite 
disc  is  rapidly  revolved,  either  between  rubbing  pads  of  silk  covered 
with  an  amalgam  of  mercury,  in  which  case  a  high  electrical  pressure  is 
generated  directly  by  the  friction,  or  in  what  is  know  as  an  electrostatic 
induction  machine,  in  which  there  are  two  ebonite  discs,  to  one  of 
which  a  small  charge  is  communicated  by  touching  it  with  the  finger, 
the  other  then  commencing  to  generate  a  pressure  which  gradually 
increases  as  the  speed  increases,  and  which  may  be  of  very  consider- 
able voltage.  The  frictional  electrical  exploder  furnishes  sufficient 
pressure  to  explode  a  comparatively  large  number  of  high  tension 
fuses,  and  is  therefore  still  rather  a  favourite  where  a  large  number 
of  shots  are  fired  together,  as  in  sinking,  and  where  a  large  quantity 
of  mineral  is  got  down  at  one  operation  in  one  place.  The  great 
objection  to  frictional  apparatus  is  its  great  sensitiveness  to  the 
presence  of  moisture  and  dust.  If  the  ebonite  discs  become  covered 
with  a  very  thin  layer  of  moisture,  as  they  are  almost  sure  to  do 
anywhere  in  a  coal  mine,  and  if,  as  would  naturally  follow,  a  deposit 
of  coal  dust  takes  place  upon  the  discs,  the  apparatus  is  rendered 
useless.  For  surface  work,  however,  where  the  apparatus  can  be  kept 
dry  and  clean,  and  where  it  is  in  the  hands  of  a  man  who  does  not 
mind  taking  a  considerable  amount  of  trouble  to  keep  it  so,  the 
apparatus  will  do  good  service. 


CHAPTER   III 
ELECTRIC  LIGHTING  FOR  MINES 

THERE  are  three  kinds  of  electric  lamps  that  are  applicable  for  use 
in  mines,  the  arc  lamp,  the  incandescent  lamp,  and  the  Nernst  lamp, 
the  latter  occupying  a  position  betweeen  the  arc  and  the  incandescent 
lamps. 

Arc  Lamps 

There  are,  again,  three  forms  of  arc  lamps  in  use,  all  of  which  can 
be  made  to  do  good  service  in  and  about  both  coal  and  metalliferous 
mines,  viz.  the  Open  Arc,  the  Enclosed  Arc,  and  the  Flame  Arc.  All 
three  lamps  are  made  also  for  use  with  both  continuous  and  alternating 
currents,  though  the  light  furnished  by  the  arc  lamp,  when  used  with 
alternating  currents,  as  will  be  explained,  is  different  to  that  when 
using  continuous  currents. 

The  enclosed  arc  lamp  is,  perhaps,  the  best  all-round  arc  lamp  for 
use  in  and  about  mines,  principally  because  it  is  simpler  in  construc- 
tion, and  because  it  burns  longer  without  attention.  All  arc  lamps, 
except  the  flame  lamps,  and  some  even  of  those,  are  arranged  upon 
the  same  general  lines.  There  are  in  nearly  every  case  two  carbons 
arranged  vertically  one  above  the  other,  the  upper  carbon  being 
always  the  positive,  or  the  carbon  from  which  the  current  passes  into 
the  arc.  In  all  forms  of  arc  lamps  except  those  known  as  shunt 
lamps,  iand  the  majority  of  the  flame  lamps,  the  upper  carbon  rests 
upon  the  lower  carbon  when  no  current  is  passing,  and  the  lamp  is 
not  burning.  When  current  is  switched  on,  the  mechanism  of  the 
lamp  separates  the  carbons  by  a  very  small  distance,  •$%  inch  in 
the  case  of  the  open  arc,  and  from  |  inch  upwards  in  the  case  of 
the  enclosed  arc,  the  current  then  jumping  the  space  between  the 
carbons  and  "  striking  the  arc,"  as  it  is  termed.  If  the  pressure 
of  the  current  is  sufficient  to  maintain  the  arc  between  the  ends  of 
the  carbon  rods,  the  lamp  continues  to  burn,  and  it  burns  as  long 
as  these  conditions  rule.  As  the  lamp  burns,  the  carbons  waste 

45 


46  ELECTRICITY   IN   MINING 

away,  partly  by  oxidation  in  the  intense  heat  produced  by  the  arc, 
about  5000°  C.,  but  more  by  the  conversion  of  the  positive  carbon 
into  vapour.  As  the  carbons  waste,  they  must  either  be  fed  towards 
each  other,  the  pressure  must  be  increased  in  proportion  to  the 
increased  distance  between  the  carbons,  or  the  lamps  must  go  out. 
In  practical  arc  lamps  the  method  adopted  is,  when  a  certain  portion 
of  carbon  has  wasted,  and  the  arc  is  therefore  longer  by  a  certain 
amount,  a  very  small  fraction  of  an  inch  in  the  case  of  a  well-con- 
structed and  well-regulated  open  arc  lamp,  the  mechanism  of  the 
lamp  causes  either  the  upper  carbon  to  approach  the  lower  one  by  a 
small  fraction  of  an  inch,  making  up  the  amount  that  has  been 
wasted,  and  possibly  a  little  more,  or  both  carbons  slightly  approach 
each  other.  A  wink  may  easily  be  seen  when  the  lamp  feeds,  as  it 
is  termed,  even  with  the  best  forms  of  lamp,  and  for  that  reason,  when 
very  good  illumination  is  required,  many  more  arc  lamps  should  be 
provided  than  would  be  necessary  to  furnish  the  light  required,  in 
order  that  the  winking  of  the  individual  lamps  may  be  masked. 
For  colliery  sidings,  pit  heaps,  even  pit  bottoms,  where  arc  lamps 
can  be  employed,  the  matter  of  the  wink  when  the  lamp  feeds  is  of 
very  little  consequence,  providing  it  burns  continuously.  In  the 
enclosed  lamp,  as  already  indicated,  the  arc  is  very  much  longer 
than  with  the  open  lamp.  In  addition,  the  carbons  burn  in  a 
different  form.  In  the  open  arc  with  continuous  current,  the 
negative  carbon  burns  to  a  sharp  point,  with  some  bubbles  of 
incandescent  carbon  usually  visible  at  different  parts.  The  positive 
carbon  burns  also  almost  to  a  point,  but  right  at  the  end  the  point 
is  blunted,  and  there  is  a  small  depression  known  as  the  crater. 
It  is  in  the  crater  where  the  conversion  of  the  positive  carbon 
into  vapour  takes  place,  and  where  the  largest  part  of  the  current 
employed  in  the  arc  lamp  is  converted  into  heat.  The  crater  also 
performs  the  very  useful  office  of  reflector.  When  the  lamp  is 
burning,  the  little  area  of  the  surface  of  the  crater  is  at  an  intense 
white  heat,  and  with  the  continuous  current  lamp  the  rays  emitted 
are  thrown  directly  downwards.  In  the  enclosed  arc  lamp  the 
carbons  burn,  both  of  them,  almost  flat.  In  addition,  the  carbons 
waste  very  much  more  slowly  than  in  the  open  arc,  the  principal 
reason  being,  as  explained  below,  that  oxidation  is  almost  entirely 
suppressed  within  a  few  minutes  of  the  lamp  being  switched  on. 
The  result  of  burning  the  longer  arc,  which  may  be  even  as  long  as 
an  inch  or  more,  though  it  then  assumes  a  very  distinctly  violet 
hue,  and  the  slow  wastage  of  the  carbons,  is  that  the  feeding  of  the 
lamp  is  very  much  less  frequent  than  with  the  open  arc,  and,  further, 
it  is  not  so  noticeable.  With  the  open  arc  the  light  rays  are  very 
much  confined,  owing  to  the  closeness  of  the  carbons  together,  and 
therefore  the  feed  is  very  apparent. 


ELECTRIC   LIGHTING  FOR   MINES 


47 


The  Mechanism  of  Arc  Lamps 

The  mechanism  of  the  enclosed  arc  lamp  is  simpler,  as  already 
explained,  than  the  open  arc  lamp,  for  the  reasons  given,  because 
the  wastage  of  the  carbon  is  less 
and  because  the  arc  is  longer.  It  is 
always  easier  to  maintain  a  long  arc 
when  there  is  sufficient  pressure,  and 
the  other  conditions  are  suitable, 
than  a  short  arc,  because  there  is 


Fia.  21.— Section  of  the 
Brockie-Pell  Single-carbon 
Arc  Lamp,  as  made  by 
Messrs.  Johnson  &  Phillips, 
showing  the  Mechanism. 


FIG.  22. — A  Section  of  the  Lamp 
shown  in  Fig.  21,  but  from  a 
Point  of  View  at  Eight  Angles. 
The  two  Solenoids  are  shown 
and  their  Gores,  with  the  Brake- 
wheel  above.  The  Globe  is 
lowered  for  Recarboning. 


more  room  to  play  with.     The  mechanism  of  open  arc  lamps  runs  on 
a  few  lines,  the  brake  mechanism  first  introduced  in  the  Brockie-Pell 


48  ELECTRICITY   IN   MINING 

lamp  a  good  many  years  back  being  the  favourite.  In  this  mechanism, 
one  form  of  which  is  shown  in  Figs.  21  and  22,  which  is  applied 
only  to  continuous  current  open  arcs,  the  upper  carbon,  and  some- 
times both  carbons,  are  suspended  from  a  band,  chain,  or  cord.  It 
is  a  very  common  practice  in  this  form  of  lamp  to  make  the  lamp 
focussing,  by  raising  or  lowering  the  lower  carbon  holder  at  the  same 
time  as  the  upper  carbon  holder  is  lowered  or  raised,  the  arc  remain- 
ing in  practically  the  same  position  with  reference  to  the  outer  globe. 
The  cord,  or  chain,  or  band  passes  round  a  wheel  supported  above 
the  platform  upon  which  the  remainder  of  the  mechanism  of  the 
lamp  is  fixed,  and  when  it  only  supports  the  upper  carbon,  the  end 
of  the  chain  or  cord  is  attached  to  the  upper  carbon  holder.  When 
both  carbons  move,  the  other  end  of  the  chain  or  cord  is  taken 
round  a  pulley  at  the  bottom  of  the  lamp,  and  is  then  attached 
to  the  lower  carbon  holder.  When  the  carbons  are  placed  in  the 
lamp  the  cord  or  chain  is  wound  up  on  its  pulley,  and  as  the  carbons 
waste  away,  the  cord  or  chain  is  gradually  unwound,  the  upper 
carbon  being  gradually  allowed  to  move  towards  the  lower  carbon, 
or  the  two  carbons  being  allowed  to  move  towards  each  other.  The 
wheel  upon  which  the  cord  or  chain  is  wound  is  controlled  by  various 
methods,  by  a  second  wheel,  by  a  sector  and  ratchet,  by  a  lever, 
and  other  arrangements ;  the  controlling  apparatus  is  worked  usually 
by  the  cores  of  two  solenoids,  one  of  which  has  its  coils  in  series 
with  the  arc,  and  is  termed  the  main  magnet,  the  other  having  its 
coils  in  parallel,  or,  as  it  is  termed,  in  shunt  with  the  arc.  The 
coils  of  the  main  solenoid  must  be  of  sufficient  sized  copper  wire 
to  accommodate  the  largest  current  the  lamp  can  be  called  upon  to 
burn  with.  The  shunt  magnet  is  wound  with  very  fine  wire.  In 
some  forms  the  shunt  is  merely  bridged  across  the  arc.  That  is  to 
say,  one  end  of  the  shunt  coils  would  be  connected  to  the  positive 
carbon  holder,  and  the  other  end  to  the  negative  carbon  holder ;  but 
in  the  great  majority  of  cases  the  shunt  coils  are  bridged  across  the 
terminals  of  the  lamp,  and  have  the  benefit  of  the  pressure  across 
the  main  solenoid  coils  as  well  as  across  the  arc.  When  the  current 
is  switched  on  to  the  lamp,  the  main  magnet  turns  the  wheel  in  one 
direction,  separating  the  carbons  to  a  given  definite  distance,  regulated 
by  screws  provided  for  the  purpose.  Before  the  arc  commences  to 
burn,  the  shunt  coil  has  practically  no  current.  When  the  arc  has 
been  struck,  the  shunt  coil  has  a  current  passing  through  it,  exactly 
in  proportion  to  Ohm's  law.  Taking  the  pressure  across  the  terminals 
of  a  lamp  as  50  volts,  the  resistance  of  the  shunt  coil  will  be  from 
750  ohms  upwards,  and  the  current  passing  through  its  coils  will 
then  be  one-fifteenth  of  an  ohm.  As  the  arc  increases  by  the 
wastage  of  the  carbons,  the  current  passing  through  the  shunt  coils 
gradually  increases,  the  pull  of  the  shunt  solenoid  upon  its  core 


ELECTRIC   LIGHTING   FOR   MINES 


49 


gradually  increases,  while  the  current  in  the  series  coils  being 
gradually  lessened  by  the  lengthening  of  the  arc,  the  pull  of  the 
series  solenoid  upon  its  core  gradually  weakens,  and  at  a  certain  point 
the  pull  of  the  shunt  overcomes  that  of  the  series,  and  the  brake- 
wheel  is  allowed  to  revolve  a  small  distance,  the  carbons  moving  that 
distance  towards  each  other,  and  this  takes  place  at  every  feed.  In  the 
Luna  arc  lamp,  made  by  the  Electrical  Company,  the  carbon  holders 
are  supported  by  a  chain  passing  over  a  sprocket  wheel  which  runs 
between  two  plates,  which  are  hinged  at  their  lower  end,  and  have 
an  armature  attached  to  the  upper  end.  The  sprocket  wheel  is  con- 
nected by  spur  gearing  with  a  train  of  wheels,  the  last  of  which  is  a 
star-escape  wheel  that  engages  with  a  steel  pallet  fixed  on  the  frame 
of  the  lamp.  When  the  star  wheel  is  locked  by  the  steel  pallet,  the 
lamp  cannot  feed,  and  the  office  of  the  shunt  magnet  in  this  lamp  is 
to  release  the  wheel  train,  the  weight  of  the  carbons  then  causing  the 
wheels  to  run,  the  carbons  to  approach  each  other, 
until  the  shunt  again  loses  its  power  and  the  train 
is  locked.  The  Luna  lamp  is  one  of  the  few  that 
is  made  either  as  a  purely  shunt  lamp,  or  as  a 
series  and  shunt,  or  differential  lamp,  as  it  is  usual 
to  term  them.  In  the  purely  shunt  lamp  there  is 
no  main  magnet,  its  place  being  taken  by  a  spring 
which  opposes  the  pull  of  the  shunt  magnet.  When 
the  lamp  is  not  burning,  the  carbons  are  separated 
by  what  will  be  the  length  of  the  arc  when  burning, 
and  the  first  effect  of  the  current  is,  the  shunt  being 
of  sufficient  power,  the  arc  being  open,  to  overcome 
the  pull  of  the  spiral  spring.  The  wheel  train  is 
unlocked,  the  carbons  run  together,  the  shunt  magnet 
then  loses  its  current  entirely,  the  spiral  spring 
separates  the  carbons,  striking  the  arc,  and  the  shunt 
comes  into  operation  again  when  the  arc  is  sufficiently 
long  to  require  a  feed.  The  office  of  the  series  magnet 
in  the  Luna  and  other  differential  lamps  is  to  separate 
the  carbons,  which  are  together  when  the  lamp  is 
not  burning,  in  the  same  manner  as  the  spiral  spring 
does. 

The  Krieg  and  Methiessen  Lamps.— Fig.  23  FIG.  23.— showing 
shows  an  open  arc  lamp  of   this   firm's  make.     In 
these    lamps,   which    are   supplied    by    the    Union 
Electrical  Co.,  a  train  of  wheels  is  also  employed, 
something   on   the   same   lines   as  the  Luna.     The 
lamps   are  made  for  shunt  and  differential,  just   as 
the  Luna  are,  the  shunt  being  opposed  by  a  spring  which  strikes  the 
arc  when  the  lamp  is  first  turned  on.     In  some  forms  of  this  lamp 


the  Mechanism 
of  one  of  the 
Union  Electri- 
cal Co.'s  Open 
Arc  Lamps. 


ELECTRICITY   IN   MINING 


there  is  a  heat  compensator,  which,  it  is  claimed,  prevents  the  arc 
increasing  beyond  a  certain  amount,  by  increasing  the  resistance  of 
the  coils.  The  open  arc  lamp  is  made  to  burn  as  much  as  twenty 
hours  twith.  single  carbons,  and  as 
much  as  forty  hours  with  double 
carbons. 

The  Enclosed  Arc  Lamps.— 
The  principal  feature  of  the  enclosed 
arc  lamp  is  the  fact  that  its  carbons 
burn  in  an  enclosure  consisting  of 


FIG.  24. — Showing  the  Mechanism  of  the 
"  Jandus  "  Enclosed  Arc  Lamp,  with 
its  Special  Form  of  Electro-magnet. 


FIG.  25.— Messrs.  Johnson  &  Phillips' 
"  Ark  "  Enclosed  Arc  Lamp.  A  is 
the  Contact  Frame  for  the  Upper 
Carbon  Bod,  B  is  the  Solenoid 
Electro-magnet,  C  is  a  Stationary 
Portion  of  the  Iron  Core  of  the 
Solenoid,  D  is  the  Moving  Portion 
of  the  Iron  Core,  E  are  Pins  con- 
necting it  to  the  Clutch  Cams, 
which  grip  the  Upper  Carbon,  F 
are  the  Contact  Pieces  through 
which  the  Current  passes. 


a  globe,  so  arranged  that  when  the  lamp  is  started,  the  heat 
liberated  by  the  arc  drives  the  air  out  of  the  enclosing  globe 
through  a  valve  provided  for  the  purpose,  the  remainder  of  the 


ELECTRIC   LIGHTING   FOR   MINES 


burning  taking  place  in  an  atmosphere  consisting  almost  entirely  of 
carbonic  oxide  gas.  It  is  this  atmosphere  which  lessens  the  waste 
of  the  carbons  by  reducing  the  oxidation  almost  to  nil.  As  explained, 
also,  it  enables  the  arc  to  be  very  much  longer.  The  carbons 
employed  must  be  very  pure,  otherwise  chemical  actions  will  be 
set  up  that  will  practically  neutralize  a  large  portion  of  the  effect 
produced  by  enclosing  the  arc.  It 
will  not  do,  for  instance,  to  use  the 
ordinary  carbons  made  for  open  arc 
lamps  in  enclosed  lamps.  The 
mechanism  of  the  enclosed  lamp 


FIG.  26.— Diagram  of  Arrangement  of  B.  T.  H. 
Co.'s  Enclosed  Arc  Lamp,  showing  the 
Mechanism,  Cut-out,  Steadying  and  Adjust- 
ing, and  Substitutional  Kesistance. 


FIG.  27.— View  of  "  Angold  " 
Single-carbon  Enclosed  Arc 
Lamp,  showing  the  Mechan- 
ism and  Adjustable  Resist- 
ance. 


consists,  in  the  simplest  forms  of  lamp,  of  a  solenoid  in  series 
with  the  arc,  the  pull  of  the  solenoid  being  opposed  by  a  spring, 
and  actuating  some  simple  form  of  clutch.  The  pressure  absorbed 
by  the  enclosed  arc  lamp  is  from  80  volts  upwards,  and  a  resistance 
is  usually  enclosed  in  the  lamp  case,  enabling  the  lamp  to  be  run 
directly  on  services  of  as  high  as  260  volts. 

Two  forms  of  enclosed  arc  lamps  demand  attention,  the  Jandus, 
shown  in  Fig.  24,  in  which  there  is  a  compound  solenoid,  and  the 


52  ELECTRICITY   IN    MINING 

Johnson  and  Phillips'  enclosed  arc  lamp,  shown  in  Fig.  25,  in  which 
there  is  also  a  special  form  of  electro-magnet.  In  the  "  Ark  "  lamp, 
which  is  the  name  given  by  Messrs.  Johnson  and  Phillips  to  their 
enclosed  lamp,  the  upper  carbon  holder  is  dispensed  with,  the  arc- 
striking  mechanism  and  the  feed  mechanism  acting  directly  upon  the 
upper  carbon  itself.  The  upper  carbon  is  gripped,  when  the  current 
is  switched  on  to  the  lamp,  by  four  clutch  cams 
contained  in  a  box,  through  the  centre  of  which  the 
upper  carbon  passes,  the  cams  being  actuated  by  a 
solenoid  of  a  special  form  in  the  lamp  case  above. 
In  addition  to  this,  there  are  four  contact  pieces, 
which  are  also  pulled  into  contact  with  the  upper 
carbon  holder  when  the  arc  is  struck,  and  it  is  there- 
fore through  these  that  the  connection  is  made  to 
the  upper  carbon.  Figs.  26,  27,  and  28  show  other 
forms  of  enclosed  arc  lamps. 

The  Lower  Carbon  Holder.— In  all  forms  of 
arc  lamp  the  lower  carbon  is  carried  in  a  holder, 
either  held  at  the  bottom  of  a  rectangular  frame 
depending  from  the  box  in  which  the  lamp  mechanism 
is  carried,  or  it  is  held  in  the  bottom  member  of  the 
frame  supporting  the  gallery  below,  and  also  the 
globe.  The  gallery  supporting  the  globes  is  some- 
times made  to  lower,  leaving  the  carbon  holder  in 
position  so  that  the  lamp  can  be  trimmed;  and  in 
other  cases  other  arrangements  are  made. 

Double  Carbon  Arc  Lamps. — Both  open  and 
enclosed  arc  lamps  are  made  by  some  firms  to  burn 
two  carbons,  the  object  being  to  give  a  long  period 
without  changing  the  carbons.     The  early  Brush  arc 
_         lamps,  it  will  be  remembered,  were  double-carbon 
Union  Etectri6-  lamps,  and  the  arrangement  employed  to-day  is  very 
cal  Co.'s  Differ-  similar  to  that  employed  in  the  Brush  lamp  of  twenty- 
ential  Enclosed  fi ve  years  ago.    In  the  double-carbon  lamp  the  two  sets 
of  carbons  are  held  vertically,  as  in  the  single-carbon 
lamp,  the  positive  carbons  resting  upon  the  negatives. 
The  arc  is  struck  by  the  same  mechanism  between  each  pair  of  carbons, 
and  the  feeding  mechanism  is  also  the  same ;  that  is  to  say,  there  is 
one  main  solenoid,  one  shunt  solenoid,  and  so  on.     It  is  in  the  clutch 
that  the  double  action  is  usually  arranged.     When  the  two  pairs  of 
carbons  are  both  in  contact,  there  are  two  paths  for  the  current, 
but  if  one  pair  is  separated,  there  remains  only  one  path  for  the 
current.     When    the    current    is    switched    on,    the    arc-striking 
mechanism  first  separates  one  pair  of  carbons,  and  then  strikes  the 
arc  between  the  other  pair.     If  the  carbons  that  are  burning  stick, 


ELECTRIC   LIGHTING   FOR   MINES 


53 


or  burn  too  long  an  arc,  the  second  pair  come  into  service,  the  arc 

being  formed  between  them  until  the  first  pair 

right   themselves.     The   usual   arrangement   for 

substituting  the   second   pair  of  carbons   when 

the  first  pair  is  burned  out  is,  the  first  pair  are 

prevented  from  feeding  onwards  after  the  carbons 

have  burned  to  a  certain  length  by  a  stop  provided 

for  the  purpose,  and  the  current  then  ceases  to  pass 

through  them,  and  is  automatically  switched  on 

to  the  other  pair. 

Figs.  29  and  30  show  double-carbon  arc  lamps. 

Dash-PotS. — In  all  arc  lamps  dash-pots  play 
an  important  part.  They  are  practically  buffers, 
and  are  intended  to  reduce  the  sometimes  quick 
action  of  the  feeding  mechanism.  A  favourite 
form  of  dash-pot  is  a  brass  cylinder  with  a  piston 
moving  inside  it ;  and  it  is  arranged  that  when 
the  arc  is  to  feed,  the  mechanism  has  to  compress 
the  air  in  the  cylinder  before  it  can  do  so,  this 
tending  to  make  the  feed  a  little  less  quick,  and 
to  give  a  better  regulation  to  the  lamp,  a  more 
steady  feed. 

Twin  Arc  Lamps.  — This  is  a  form  of  double 
arc  lamp  which  has  been  introduced  since  the 
supply  pressures  in  most  towns  has  been  increased 
to  200  volts  and  upwards.  It  consists  of  two 
separate  sets  of  carbons  through  which  the  current 
passes  in  series,  so  that  the  whole  of  the  double 
pressure  is  used  up.  The  twin  arc  lamps  are 
more  commonly  employed  with  enclosed  lamps 
than  with  open  arcs,  as  the  pressure  of  the 
enclosed  lamp  with  two  arcs  practically  uses  up 
the  pressure  of  the  service. 

Small  Arc  Lamps. — Nearly  all  the  makers 
of  arc  lamps  now  make  very  small  lamps  under 
various  names,  such  as  the  "  Midget,"  that  may 
be  of  service  in  certain  parts  of  a  mine,  such  as 
the  engine-house,  the  heapstead,  fitting  shops, 
large  pumping-houses,  etc.  They  are  made  to 
take  very  small  currents,  but  with  the  same 
pressure  as  with  the  larger  forms  of  arc  lamps, 
and  they  give  very  much  less  light  in  propor- 
tion to  the  larger  lamps.  They  are  made  in 
various  forms  for  currents  from  1  ampere  to 
3  amperes,  and  for  open  and  enclosed  arcs. 


:G.  29.  — "Angold  " 
Open-type  Double- 
carbon  Arc  Lamp. 


54  ELECTRICITY   IN   MINING 


Alternating  Current  Arc  Lamps 

The  alternating  current  arc  lamp  is  similar  in  a  great  many 
respects  to  the  continuous  current  lamp,  but  the  light  given  in  the 
case  of  all  but  the  flame  arc  lamps  with  converging  carbons  is 
different.  In  all  alternating  current  arc  lamps  with  carbons  arranged 
vertically  one  above  the  other,  both  carbons  are  partially  pointed, 
both  have  their  points  partly  blunted,  and  both  have  small  craters, 
approximately  half  the  size  of  the  crater  formed  in  the  positive 
carbon  with  continuous  current  lamps.  The  reason  is,  both  carbons 
are  alternately  positive  and  negative.  The  result  is  that  in  place  of, 
as  in  the  continuous  current  arc,  the  light  being  thrown  downwards 
from  a  line  30°  from  the  vertical  line  passing  through  the  arc, 
part  of  the  light  is  thrown  upwards,  and  part  downwards.  The 
carbon  holder  and  frame,  etc.,  below  the  arc  stop  the  light 
immediately  below,  and  the  case  containing  the  lamp  mechanism 
stops  it  above,  and  the  result  is  that  there  are  two  lighted  areas 
with  the  enclosed  arc  lamp,  extending  approximately  from  a 
line  about  15°  from  the  horizontal  to  30°  from  the  vertical  above 
and  below  the  arc,  while  with  a  continuous  current  the  light  only 
extends  over  that  area  below  the  arc.  The  arc  lamp  mechanism  is 
specially  constructed  for  working  with  alternating  currents,  the 
important  feature  being  the  cores  of  the  electro-magnets,  and  any 
moving  iron  that  takes  part  in  the  action  of  the  lamp,  which  are  very 
carefully  laminated ;  that  is,  they  are  split  up  into  a  number  of  thin 
plates,  and,  where  possible,  of  wires.  The  alternate  current  arc  lamp 
burns  with  a  much  lower  pressure  than  the  continuous  current.  It 
is  usual  to  allow  50  volts  per  lamp  for  continuous  current  lamps, 
where  about  38  volts  is  sufficient  with  alternating  current  lamps. 

Flame  Arc  Lamps 

The  flame  arc  lamp  is  a  recent  introduction,  and  its  special 
feature  is  the  colour  of  the  light  furnished  by  the  lamp,  and  the 
peculiar  flame  with  which  it  burns.  In  the  great  majority  of  cases, 
also,  the  lamp  is  made  with  its  carbons  suspended  from  the  case 
containing  the  lamp  mechanism,  the  two  carbons  being  slightly 
inclined  to  the  vertical  and  to  each  other.  The  Electrical  Co.,  how- 
ever, make  a  flame  arc  lamp  in  which  the  carbons  are  suspended 
vertically  one  above  the  other,  as  in  the  ordinary  lamp,  as  well  as 
the  type  with  converging  carbons.  The  colour  of  the  light  is  due  to 
the  carbons,  in  the  course  of  preparation,  having  been  impregnated  with 
salts  of  sodium,  calcium,  and  strontium.  Carbon  rods  for  use  in  arc 
lamps  are  made  by  grinding  up  charcoal  and  the  purest  forms  of  carbon 


ELECTRIC   LIGHTING   FOR   MINES 


55 


obtainable  to  a  very  fine  powder,  kneading  the  powder  into  a  paste 
with  a  glutinous  substance,  and  forming  the 
carbons  by  pressing  them  through  dies,  the 
carbons  being  afterwards  baked  in  retorts. 
In  the  carbons  employed  in  flame  arc  lamps, 
the  salts  of  sodium,  calcium,  and  strontium 
are  introduced  with  the  glutinous  material. 
In  one  form  also,  those  used  in  the  "  Excello," 
made  by  the  Union  Electrical  Co.,  a  thin 
wire  is  also  introduced  into  the  carbon  to 
lessen  its  resistance.  In  several  of  the  flame 
arc  lamps  the  ends  of  the  two  carbons  pro- 
ject downwards  into  an  inverted  basin  of 
lime  or  a  similar  substance,  which  performs 
the  office  of  a  reflector  and  condenser.  When 
the  lamp  is  not  burning,  the  ends  of  the 
two  carbons  either  rest  against  each  other, 
or  the  two  ends  rest  against  a  piece  of  iron 
which  completes  the  circuit.  When  the  cur- 
rent is  switched  on,  either  an  electro-magnet, 
or  some  equivalent  apparatus,  either  pulls 
one  carbon  away  from  the  other,  pulls  the 
two  carbons  in  opposite  directions,  or  pulls 
the  piece  of  iron  away  from  the  ends  of 
the  carbon.  In  either  case  the  arc  is  then 
struck  between  the  ends  of  the  carbon  rods 
in  the  usual  way,  but  it  is  a  horizontal  and 
not  a  vertical  arc,  as  in  the  ordinary  form 
of  lamp.  In  addition  to  this,  in  all  forms 
of  flame  arc  lamp  there  is  an  electro-magnet, 
whose  office  is  to  repel  the  arc  formed 
between  the  carbons.  One  important  pro- 
perty of  the  electro-magnet  which  has  re- 
ceived very  little  attention,  and  very  little 
use  previously  to  the  introduction  of  the 
flame  arc,  is  its  ability  to  repel  the  arc. 
In  the  flame  arc  lamp  the  repulsion  by  the 
electro-magnet  spreads  the  flame  out  in 
the  form  of  a  fan,  and  this,  aided  by  the 
reflector  above,  gives  the  lamp  itself  the 
appearance  of  a  globe  of  orange-coloured  FIG.  30.-"  Angol. 
flame.  It  will  be  noticed  that  there  are, 
in  the  majority  of  flame  arc  lamps,  no  objects 
below  the  arc,  as  there  are  in  the  other  forms  of  arc  lamps,  to  cast 
shadows,  and  therefore  a  very  much  better  lighting  effect  is  produced. 


carbon  Open- type  Arc  Lamp 
for  Rectified  Current. 


ELECTRICITY   IN    MINING 


It  is  also  claimed  that  the  flame  arc  gives  a  larger  quantity  of  light 
for  a  given  expenditure  of  electrical  energy  than  either  of  the  other 
forms  of  arc  lamps.  Fig.  31  shows  the  "  Ex  cello "  flame  arc  lamp. 
Fig.  32  shows  the  larger  light  claimed  for  a  flame  arc. 

The  Juno  Flame  Arc  Lamp.— As  explained,  in  the  great 
majority  of  flame  arc  lamps,  the  striking  mechanism  is  an  electro- 
magnet acting  either  directly  or  through  t  he  mechanism  common  to 
the  particular  type  of  lamp.  Thus,  in  the  Excello  flame  lamp, 
the  Krieg  and  Mathiessen  form  of  mechanism  has  been  maintained 


FIG.  31. — Sections  of  the  "  Excello  "  Flame  Arc  Lamp,     h  is  the  Main  Electro 
magnet,  n  the  Shunt-magnet,  b  and  d  are  the  Arc  Mechanism. 

for  striking  the  arc ;  also,  in  the  Luna  flame  arc  lamp  the  mecha- 
nism employed  in  the  ordinary  Luna  lamps  has  also  been  used. 
Messrs.  Johnson  &  Phillips,  however,  have  introduced  a  distinctly 
novel  form  of  mechanism  for  striking  the  arc,  and  in  doing  so  they 
have  been  enabled  to  simplify  the  apparatus.  In  the  Juno  flame 
lamp  the  striking  mechanism  is  operated  by  the  expansion  of  a  nickel 
steel  wire,  when  the  current  passing  to  the  arc  goes  through  it.  The 
arrangement  is  a  very  simple  one.  One  carbon  lies  against  the 
other  when  the  current  is  not  switched  on.  When  the  current  passes, 


ELECTRIC   LIGHTING   FOR   MINES 


57 


the  nickel  steel  wire  expands,  and  in  expanding  operates  a  trip  lever 
which  pulls  the  movable  carbon  away  from  its  fellow,  the  length  of 
the  arc  being  regulated  by  screws  provided  for  the  purpose.  The 
spreading  electro-magnets  are  two  iron  rods,  which  are  used  to  sup- 
port the  lower  part  of  the  lamp  where  the  reflector  is  placed,  with 
insulated  wires  wound  round  them,  the  current  passing  through  these 
wires  on  its  way  to  the  arc.  The  feeding  mechanism  is  also  very 
simple,  one  carbon  simply  slides  down  against  a  projection  arranged 
for  it,  and  the  other  carbon  slides  through  a  tube  as  far  as  it  is 
allowed  by  the  arc-striking  mechanism.  The  arrangement  and  con- 


FIG.  32.— Showing  the  larger  Quantity  of  Light  given  by  Flame  Arcs, 
as  claimed  for  the  "  Excello  "  Lamp.  The  Dotted  Line  shows  the 
Light  given  by  an  Ordinary  Arc,  the  Thick  Line  that  claimed  for 
the  "Excello"  Lamp.  The  Candle-powers  are  shown  by  the  Figures 
at  the  Ends  of  the  Curves,  and  the  Angles  by  the  Radii. 

nections  are  shown  in  Fig.  33.  The  author  has  introduced  the 
description  of  the  flame  arc  lamp,  though  he  is  not  aware  that  it 
has  yet  been  employed  in  mining  work,  because  it  appears  to  him 
that  it  would  be  of  great  service  in  coal  screening  at  night.  One  of 
the  difficulties  of  properly  cleaning  the  coal  by  artificial  light,  as  he 
understands,  is  that  some  kinds  of  shale  and  dirt  have  very  much  the 
same  appearance  as  coal,  when  viewed  by  artificial  light.  All  forms 
of  artificial  light,  in  the  author's  experience,  are  rich  in  the  red  rays 


58  ELECTRICITY   IN   MINING 

of  the  spectrum,  and  it  is  for  this  reason  that  the  difficulty  arises  on 
the  picking  belts.     The  flame  arc  lamp,  providing  that  proper  salts 


FIG.  33. — "  Juno  "  Flame  Arc  Lamp.  A  is  a  Copper  Best  for  the  Stationary- 
Carbon,  B  is  a  Metal  Cap  shielding  the  Arc,  C,  C2  are  the  Carbons,  D  is 
a  Screw  holding  A  to  B,  E  is  an  Insulated  Brass  Plate,  F,  G,  and  K  is 
the  Apparatus  for  striking  the  Arc,  W  is  the  Nickel  Steel  Wire  which 
heats,  RE  are  the  Electro-magnetic  Kods  which  spread  the  Arc. 

are  used  in  impregnating  the  carbons,  is,  in  his  experience,  the  only 
exception.     The  light  given  by  the  flame  arc,  in  which  the  salts  of 


ELECTRIC   LIGHTING   FOR   MINES  59 

sodium  are  predominant,  gives  light  practically  the  same  for  colour 
purposes  as  sunlight.  The  author  has  tested  every  form  of  artificial 
light  for  colour. 

Delivering  Current  to  the  Arc  Lamps.— The  simplest  arrange- 
ment for  delivering  continuous  current  to  arc  lamps  where  the  service 
is  either  110,  220,  or  500  volts  is  by  connecting  two  or  more  lamps 
in  series,  the  series  being  connected  between  the  supply  cables. 
Fig.  34  shows  a  diagram  for  lamps  from  2  upwards.  As  explained, 
two  open  arc  lamps  work  from  110  volt  services,  and  may, 
by  careful  regulation,  be  made  to  work  from  a  100  volt  service. 
Four  open  arc  lamps  work  in  series  across  a  220  volt  service,  eight 
across  a  440  volt  service,  and  from  nine  to  ten  across  a  500-volt 


FIG.  34. — Diagram  of  Connections  for  Two  and  for  Four  Arc  Lamps,  between 
200  to  220  Volt  Service  Mains. 

service.  A  simpler  arrangement,  however,  and  one  that  will  pro- 
bably prove  more  economical  where  a  varying  number  of  lamps  are 
required  to  be  in  use  together,  would  be  the  employment  of  a  motor 
generator,  in  the  same  manner  as  described  in  connection  with 
electric  signals,  the  generator  of  the  motor  generator  being  con- 
structed to  furnish  65  or  110  volts,  as  convenient.  Where  there  are 
a  number  of  arc  lamps  in  use,  if  some  of  them  are  not  required  at 
certain  times,  as  may  very  frequently  happen,  the  current  that 
would  be  employed  in  them  must  be  wasted,  or  the  carbons  in 
the  lamp  must  be  burned  uselessly.  By  the  arrangement  of  the 
motor  generator,  one  or  two  lamps  can  be  arranged  to  be  taken  off 
the  service  at  any  point  required,  and  each  lamp  may  be  switched  in 
and  out  as  and  when  convenient.  Fig.  35  shows  the  arrangement 


6o 


ELECTRICITY   IN    MINING 


for  this.  The  arrangement  has  also  the  great  advantage  that  where 
the  service  is  three-phase  alternating,  the  arc  lamp  and  incandescent 
lamp  service  also,  if  required,  may  be  kept  quite  independent  of  the 
power  service,  while  full  advantage  is  taken  of  the  three-phase 
distribution. 

Delivering  Alternating  Current  to  Arc  Lamps.  —  Arc  lamps 
employing  alternating  currents  may  be  connected  across  a  100, 
110,  220,  440,  500,  or  practically  any  alternating  current  service 
in  series,  just  as  continuous-current  lamps  are,  but  with  the  advan- 
tage explained  above,  that  the  pressure  taken  by  the  lamps  is 
smaller,  three  lamps  being  sometimes  worked  on  110  volt  service, 


OR  oaeotura   PRESSURE   SUPPLY 


MOTOR  GENERATOR 


Low  FISSURE  5eRV»cE  FOR 

ARC5   OR  INCANDESCENT 


FIG.  35. — Diagram  of  Arrangement  for  running  Arcs,  or  Incandescent  Lamps, 
from  a  high-pressure  Service,  by  the  Aid  of  a  Motor  Generator. 

and  with  the  advantage  also  that  the  choking  coil  or  compensator 
arrangement  enables  one  or  more  lamps  of  a  series  to  be  in  use 
without  very  much  waste  of  current  in  the  compensating  apparatus. 
Arc  lamps  may  also  be  worked  individually  from  a  single-phase 
alternating  current  service  by  fixing  a  transformer  to  each  lamp,  the 
transformer  being  connected  across  the  supply  service.  Alternating 
current  lamps  may  also  be  worked  on  two-phase  and  three-phase 
systems,  the  lamps  being  either  bridged  in  series  between  the  pairs 
of  cables  in  the  two-phase  system,  and  between  any  two  cables  in  the 
three-phase  system,  or  being  supplied  by  transformers  taking  current 


ELECTRIC   LIGHTING   FOR   MINES 


61 


from  two  of  the  cables  in  either  system.  The  alternating  current 
lamp,  it  will  be  understood,  only  works  with  single-phase  currents, 
and  can  therefore  only  be  used  on  two  and  three  phase  services  by 
connecting  them  either  singly  or  in  series  in  each  of  the  phases. 
Where  many  arc  lamps  are  employed,  taking  current  from  a  two  or 
three  phase  service,  they  should  be  distributed  equally,  as  far  as 
possible,  between  the  phases,  whether  they  are  supplied  directly  from 
the  two  or  three  cables,  or  through  transformers.  It  is  important, 


ARC 


LAMPS 


FIG.  36.— Diagram  of  Connections  of  Arcs  in  Parallel  across  a  65  or  100  Volt 
Service.  Each  Arc  has  its  own  Resistance  as  shown.  This  is  the 
Arrangement  that  would  be  employed  where  Current  was  taken  for  the 
Lighting  Service,  from  the  Power  Service,  through  a  Motor  Generator. 

with  two  and  three  phase  working,  that  the  same  current  strength 
shall  be  passing  in  the  two  and  three  phases.  Apparently  a  small 
difference  in  one  phase  does  not  make  any  difference  in  the  working 
of  a  well-designed  and  well-made  two  or  three  phase  generator,  but 
it  must  lower  the  whole  efficiency  of  the  system.  Where  several 
arc  lamps  are  to  be  run  in  series,  rectified  alternate  currents  are 
sometimes  employed  with  success. 


Difference  in  the  Working  of  Continuous  and 
Alternating  Current  Arc  Lamps 

The  important  difference  between  the  working  of  arc  lamps 
supplied  with  continuous  current  and  with  alternating  currents, 
apart  from  the  distribution  of  the  light,  is,  it  is  necessary  with 
continuous  current  lamps  usually  to  insert  a  resistance  in  the  circuit, 


62 


ELECTRICITY   IN   MINING 


in  order  that  the  lamp  may  strike  its  arc.  With  continuous  current 
lamps,  also,  it  is  usual  to  provide  automatic  cutouts  to  bridge  the 
lamp  over,  in  case  the  carbons  stick.  With  alternating  currents 
both  of  these  offices  are  performed  by  what  are  variously  termed 
choking  coils,  compensators,  and  by  other  names. 

With  the  continuous  current  arc  lamp,  except  in  the  case  where  the 
armature  controlling  the  striking  mechanism  moves  through  a  short 
distance,  and  is  then  locked  by  contact  with  the 
poles  of  the  electro-magnet,  if  the  full  current 
is  allowed  to  pass  into  the  lamp  when  it  is 
switched  on,  it  will  not  strike  its  arc,  but  the 
upper  carbon,  or  the  two  carbons  where  both 
move,  will  perform  the  operation  known  as 
pumping.  The  carbons  will  be  separated  by  a 
distance  too  great  for  the  arc  to  form.  They 
will  then  return  into  contact,  be  again  separated, 
return  to  contract,  and  so  on.  The  reason  is, 
the  solenoids  which  are  so  generally  employed 
with  arc  lamps  have  a  very  long  stroke,  and  the 
pull  of  a  solenoid  coil  upon  its  iron  core  increases 
very  rapidly  as  the  core  passes  into  the  coil. 
The  pressure  required  in  an  arc  lamp  is  made 
up,  partly  of  the  pressure  required  to  drive  the 
current  through  the  solenoid  coils,  the  carbons, 
and  the  resistance  of  the  arc,  but  the  greater 
portion  is  required  to  overcome  the  back  pressure 
created  in  the  arc  itself,  at  the  crater.  When 
the  current  is  first  switched  on,  the  back  pressure 
is  absent,  and  therefore  a  very  powerful  current 
passes  through  the  solenoid  coils,  producing  a 
very 'quick  action  upon  the  solenoid  core,  separ- 

FIG     37. -Automatic  atinS  the  carbons  verv  1uicklv>  but  producing 

Cutout  for  Continuous  n°  arc.     The  remedy  for  this  is,  if  the  lamp  is 

Current  Arc   Lamp,  burning  directly  from  a  supply  service,  as  was 

That  ishetoRteaketa?he  c<>mm°n  in  th^  early  days  of  lighting,  a  resistance 

Place  of  the  Arc,  as  is  inserted  in  the  circuit  sufficient  to  reduce  the 

made  by  the  General  current,   and  an  additional  pressure  is  provided 

ilectric  Co.,  London.  to  overcome  this  resistance,  the  figures  for  a 

10  ampere  lamp  being  a  resistance  of  15  ohms, 

and  the  increase  of  the  pressure  from  50  to  65  volts.     This  would  be 

the  arrangement  where  current  is  taken  from  a  motor-generator,  and 

is  shown  in  Fig.  36.    With  modern  arrangements  two  open  arc  lamps 

are  usually  run  in  series  on  a  110  volt  service,  a  resistance  absorbing 

the  10  volts,  or  four  open  arc  lamps  are  run  on  a  220  volt  service  or 

upwards,  the  arrangement  of  the  resistance  being  suited  to  meet  the 


ELECTRIC   LIGHTING  FOR   MINES  63 

pressure  of  the  service.  With  enclosed  arc  lamps  one  lamp  can  be 
run  directly  on  a  110  volt  service,  or  two  on  a  220  volt  service,  an 
adjustable  resistance  being  included  in  the  lamp-case  to  absorb  the 
pressure  over  and  above  that  required  for  the  arc.  As  explained,  the 
enclosed  arc  absorbs  from  70  to  80  volts.  The  arrangement  of  two 
lamps  in  series  with  open  arcs,  together  with  a  resistance,  and  one 
lamp  with  a  resistance  with  enclosed  arcs,  meets  the  case  of  the 
necessary  reduction  of  the  current  when  the  lamp  is  started. 

Continuous  Current  Automatic  Lamp  Cutouts.  —  Where 
either  open  or  enclosed  arc  lamps  are  run  in  series  some  provision 
is  necessary  in  case  one  of  the  lamps  refuses  to  burn,  either  by  its 
carbons  remaining  in  contact,  and  the  back  pressure  of  its  arc  being 
entirely  absent,  or,  as  is  more  common,  the  feed  mechanism  not 
acting  properly,  and  the  arc  being  extinguished  through  the  carbons 
being  allowed  to  burn  too  far  apart.  The  usual  arrangement  with 


Safety  Coils. 


FIG.  38.— Diagram  of  Alternate  Current  Arc  Lamps,  connected  in 
Series,  with  Choking  or  Balancing  Coils,  as  arranged  by  the 
Electric  Co. 

continuous  current  arcs  is,  an  auxiliary  electro-magnet,  of  which 
one  form  is  shown  in  Fig.  37,  and  is  fixed  either  in  the  lamp-case 
itself,  or  in  some  convenient  position  in  the  neighbourhood,  the  coils 
of  the  magnet  being  wound  with  two  wires,  one  a  very  fine  wire, 
usually  in  series  with  the  shunt  coils  of  the  lamp  and  receiving 
the  same  current  as  they  do,  the  other  a  thick  wire  sufficiently 
large  to  carry  the  whole  current.  The  armature  of  the  electro- 
magnet carries  a  contact  which,  when  the  shunt  coils  of  electro- 
magnet have  acquired  sufficient  power  to  overcome  the  tension  of 
the  opposing  spring,  makes  connection  with  a  contact  piece  to  which 
is  connected  a  resistance  sufficient  to  absorb  the  whole  of  the  pressure 
taken  by  the  lamp  when  burning. 

Choking  Coils  and  Compensators. — With  arc  lamps  employing 
alternating  currents,  what  are  termed  choking  coils  take  the  place 
of  the  adjustable  resistances  mentioned  in  connection  with  continuous 


64 


ELECTRICITY  IN   MINING 


S.P  Switch 0 


current  arcs,  and  the  automatic  cutout  is  replaced  by  some  form  of 
what  are  known  as  compensators.     The  choking  coil  is  merely  a 

short  coil  of  wire  of  sufficient  thickness 
to  carry  the  current  for  the  arc,  coiled 
round  a  laminated  iron  core.  Choking 
coils  are  made  in  various  forms.  They 
are  really  electro-magnets,  and  they 
are  made  sometimes  with  one  leg, 
AS.R  switch  sometimes  with  two,  and  with  the 

scnantor*  7 

MWVWWVWWMAMA  magnetic  circuit  of  the  coil  completed. 

In  either  case  the  self-induction  of  the 
current  in  the  coils  surrounding  the 
iron,  due  to  the  variations  and  reversals 
of  the  alternating  current,  create  an 
opposing  pressure,  and  "  choke  "  back 
the  current,  using  up  the  surplus  pres- 
sure in  the  same  manner  as  the  resist- 

FIG.  39.— Diagram  of  Connections  ance  coils  use  up  the  surplus  pressure 
for  Working  One  or  Two  Arc  with  continuous  currents.  The  choking 

coils  are  made  adjustable  either  by 
connecting  different  lengths  of  coil  in 
the  circuit,  or  by  adjusting  the  position 
of  the  iron  coil.  The  choking  coil  is 

inserted  in  the  main  circuit  of  the  lamp  it  is  to  "regulate,  and  it  absorbs 


Lamps  from  a  200  Volt  Alternate 
Current  Service  without  Appreci- 
able Waste,  as  arranged  by  the 
British  Thomson-Houston  Co. 


-  P    X^=~ !  .  ^^?====S7 


FIG.  40. — "Economy  Coil,"  or  "Compensator,"  made  by  the  General 
Electric  Co.,  for  Use  with  Alternate  Current  Circuits.  It  is 
regulated  by  moving  the  Coils  on  the  Left. 

a  very  small  quantity  of  energy.    Some  energy  is  absorbed,  but  it  is  only 
a  fraction  of  that  which  would  be  absorbed  by  another  lamp.     The 


ELECTRIC   LIGHTING  FOR   MINES  65 

arrangement  is  extended  to  enable  two  or  more  lamps  to  be  worked 
from  100  or  200  volt  service,  but  so  that  any  individual  lamp  can 
be  burned  by  itself  with  only  a  small  expenditure  of  electrical 
energy  outside  of  the  lamp  in  the  balancing  coil  or  compensator,  as 
they  are  called.  Diagrams  of  the  arrangement  for  working  with 
compensators  or  balancing  coils  are  shown  in  Figs.  38  and  39. 
Fig.  40  shows  one  form  of  a  "  choking  coil." 

Supporting  Arc  Lamps 

The  mechanism  of  all  arc  lamps  is  carried  by  a  brass  platform, 
usually  circular,  and  is  also  usually  covered  by  a  brass  disc  above, 
and  is  protected  by  copper  or  iron  or  brass  cylinders,  made  to  slide 
one  over  the  other,  and  to  completely  enclose  the  mechanism.  The 
lamp  is  always  suspended  by  a  glazed  earthenware  or,  perferably, 
a  porcelain  reel  insulator,  held  in  a  small  bracket,  secured  to  the  top 
of  the  lamp  so  that  the  arrangement  is  insulated  from  all  the  lamp 
connections,  as  shown  in  Figs.  23,  24,  and  others.  The  lamp  may  be 
supported  from  any  convenient  position  by  any  convenient  method, 
using  the  reel  insulator  for  the  purpose.  A  convenient  method  is,  a 
flexible  galvanized  stranded  rope  is  used  for  a  support,  one  end  being 
secured  to  the  reel  insulator,  and  the  other  end  taken  over  a  pulley  in 
the  position  to  which  the  lamp  is  to  be  hoisted,  and  from  there,  if 
necessary,  over  other  pulleys  to  a  small  winch  fixed  in  any  convenient 
place.  For  pit-heaps,  the  first  pulley  round  which  the  galvanized 
strand  is  taken  may  be  secured  to  one  of  the  longitudinal  members 
of  the  pit-heap  cover,  the  strand  being  then  taken  down  over  other 
pulleys  secured,  to  other  parts  of  the  structure  to  a  winch  at  the  side. 
A  similar  arrangement  is  suitable  for  engine-houses  where  arc  lamps 
are  employed,  and  for  pit  bottoms  if  an  arc  is  employed  there.  For 
sidings  and  open  spaces  about  the  mine,  poles  may  be  employed. 
If  they  are  of  wood  they  should  be  creosoted,  as  a  creosoted  pole 
lasts  from  five  to  six  times  as  long  as  one  not  creosoted.  Un- 
creosoted  poles  are  attacked  at  the  surface  of  the  ground,  and  in  the 
course  of  four  or  five  years  will  become  so  rotten  there  that  a  storm 
will  blow  them  down,  with  damage  to  the  lamp  and  other  trouble. 
A  good  strong  creosoted  pole  with  creosoted  arms  bolted  to  its  top, 
to  which  pulleys  are  fixed,  forms  a  very  convenient  arrangement, 
the  winch  for  the  galvanized  rope  being  fixed  near  the  bottom 
of  the  pole.  The  pole  should  be  stayed,  and  the  wires  leading  to 
the  lamp  should  be  fixed  to  insulators  carried  on  arms,  also 
bolted  to  the  top  of  the  pole  above,  and  well  clear  of  the  hoisting 
mechanism.  Iron  lamp-posts  are  better  than  wooden  posts,  and  are 
not  very  much  more  expensive.  The  base  of  the  iron  lamp-post 
is  made  hollow,  usually  square,  and  the  winch  for  the  hoisting 

F 


66 


ELECTRICITY   IN    MINING 


rope  is  fixed  inside  the  base,  but  can  be  arranged  to  be  worked  from 
the  outside  by  allowing  the  axle  of  the  winch  to  project  through  a 
hole  in  the  side.  It  is  better,  however,  to  enclose  the  whole  thing. 
The  base  of  the  iron  lamp-post  also  provides  a  convenient  place  to 
fix  automatic  cutouts,  choking  coils,  compensators,  etc.  Where  a 
lamp-post  is  used,  the  galvanized  iron  rope  is  carried  up  inside  the 

post,  the  upper  part  of  which  is 
made  of  a  straight  tube,  and  the 
actual  support  of  the  lamp  is  also 
a  tube  bent  to  any  convenient  form. 
Lattice-work  poles  are  also  used,  as 
shown  in  Fig.  41.  They  are  light  and 
strong. 

Contact  Supports  for  Arc 
Lamps  —  The  problem  involved  in 
trimming  and  cleaning  arc  lamps  is 
a  more  or  less  troublesome  one, 
principally  because  of  the  leads.  If 
the  cables  leading  to  the  lamp  are 
made  sufficiently  long,  to  allow  the 
lamp  to  be  lowered  to  within  reach 
of  the  ground,  they  sag  about  when 
the  lamp  is  in  its  position  for  lighting, 
particularly  when  there  is  much  wind, 
and  the  insulation  is  liable  to  be 
damaged.  These  considerations  have 
led  to  the  development  of  a  series  of 
contact  arrangements  for  arc  lamps, 
which  avoid  the  necessity  of  providing 
the  dangerous  lengths  of  loose  leads. 
The  bracket  which  supports  the  lamp 
carries  a  hood,  in  which  contact  springs 
or  equivalent  arrangements  are  fixed, 
the  ends  of  the  cables  from  the 
supply  service  being  connected  to 
these  springs.  On  the  top  of  the 
Fia.  41.—  Lattice-work  Iron  Lamp-  lamp-case  are  also  springs  or  plates 
post  for  Arc  Lamp,  with  Winch,  forming  the  terminals,  and  when 
Galvanized  Kope,  etc.  to  of  ^ 


lamp  passes  into  the  hood  mentioned 

above,  the  terminal  plates  or  springs  making  connection  with  the 
terminal  springs  or  plates  in  the  hood.  Several  devices  have  also 
been  introduced  complementary  to  these,  designed  to  relieve  the 
hoisting  mechanism  of  the  weight  of  the  lamp  after  it  is  hoisted 
into  position.  In  the  apparatus  made  by  the  General  Electric 


ELECTRIC  LIGHTING  FOR   MINES 


67 


Co.,  the  weight  of  the  lamp  assists  in  making  good  connection 
between  the  lamp  terminals  and  those  under  the  hood.  After 
the  lamp  is  hoisted  into  its  place,  the  springs  on  the  lamp-case 
spread  out  and  engage  with  the  contact  pieces  on  the  hood.  In  the 


FIG.  42.— Section  of  Shaeffer  Safety 
Hook  for  Arc  Lamps.  The  Ring 
shown  at  the  Bottom  is  fixed 
round  the  Insulator  on  Top  of 
the  Lamp.  This  Apparatus  is 
only  designed  to  support  the 
Lamp  so  that  it  can  be  easily 
lowered.  It  has  no  Contact 
Arrangements. 


FIG.  43. — Another  Form  of 
Shaeffer  Contact  Support  for 
Arc  Lamps.  In  this  Apparatus 
the  Lamp  is  supported  and 
Contacts  made  as  well. 


Shaeffer  apparatus,  shown  in  Figs.  42  and  43,  there  is  a  steel  tube 
and  three  steel  balls,  the  tube  pushing  the  balls  out  of  the  way 
as  it  rises  when  the  lamp  is  being  hoisted  into  position,  and 
afterwards  resting  on  them.  In  the  L.E.F.  apparatus,  a  pin  on  the 


68  ELECTRICITY   IN   MINING 

apparatus  attached  to  the  lamp  slips  into  a  socket,  in  which  it  is 
locked,  the  weight  of  the  lamp  then  being  taken.  In  either  case,  a 
half  turn  or  so  of  the  winch  raises  the  lamp  a  little  off  its  support, 
and  enables  it  to  be  lowered. 


Incandescent  Lamps 

There  are  three  types  of  incandescent  lamp  that  are  now  available 
for  use  in  mines,  and  that  should  do  good  work  there.  The  ordinary 
carbon  filament  lamp,  giving  a  light  up  to  50  C.P.,  the  high  C.P.  carbon 
filament  lamp,  giving  a  light  from  100  up  to  500  C.P.,  and  the  metallic 
filament  lamp.  The  small  carbon  filament  lamp  is  too  well  known  to 
require  description.  Its  filament  is  made  from  cellulose,  squirted 
through  a  dye,  and  formed  in  that  manner  into  a  long  thread,  which 
is  cut  into  lengths,  baked  in  retorts,  fixed  in  the  familiar  pear-shaped 
globes,  from  which  the  air  is  exhausted,  and  built  up  into  the  steel- 
looking  filament  we  know,  by  the  precipitation  of  carbon  from  coal 
gas,  the  lamp  globe  being  filled  with  the  gas  during  what  is  called  the 
flashing  process.  The  ends  of  the  filament  are  connected  to  small 
platinum  wires,  by  different  methods  of  jointing ;  a  favourite  one  being, 
a  small  sleeve  is  made  in  the  end  of  the  platinum  wire,  the  end  of 
the  carbon  filament  is  inserted  in  the  sleeve,  a  special  paste  is  put 
in  with  it,  and  the  whole  is  welded  together  by  heat.  Platinum 
is  employed  because  it  has  the  same  coefficient  of  expansion  as  glass, 
and  the  platinum  wires  are  sealed  into  the  base  of  the  pear-shaped 
globe  before  the  filament  is  connected  to  them.  The  base  of  the  pear- 
shaped  globe  is  now  almost  universally  enclosed  in  a  brass  cap,  con- 
sisting of  a  cylinder  of  thin  brass  surrounding  the  neck  of  the  lamp, 
the  space  being  filled  with  a  cement  that  does  not  absorb  moisture, 
and  in  which  one  or  two  plates  are  embedded  for  the  lamp  connections. 
In  the  B.C.  lamp  (bottom  cap)  there  are  two  semicircular  brass  plates 
on  the  bottom  of  the  cap,  bedded  in  the  cement,  and  having  the  ends 
of  the  platinum  wires  connected  to  them.  In  the  C.C.  (central  contact) 
lamp  there  is  one  circular  brass  plate,  fixed  centrally  in  the  cement 
enclosed  by  the  cylinder  of  the  cap,  having  one  of  the  platinum  wires 
connected  to  it,  the  other  being  connected  to  the  cylinder  of  the  cap 
itself.  The  outside  of  the  cylinder  of  the  cap  is  fitted  with  two  pins 
which  engage  in  the  bayonet  joint  holder,  so  universally  employed.  The 
small  carbon  filament  lamp  is  sometimes  fitted  with  a  cap  of  a  substance 
called  "  vitrite,"  a  substance  that  is  an  insulator,  and  which  is  claimed 
not  to  readily  absorb  moisture,  and  which  can  also  be  moulded  into 
any  form  required.  The  "vitrite"  cap  is  of  the  same  shape  as  the  brass 
cylindrical  cap,  the  "  vitrite  "  filling  up  the  whole  space,  the  brass  con- 
tact plates  lying  on  its  surface  at  the  bottom  of  the  cap,  the  platinum 


ELECTRIC   LIGHTING   FOR   MINES  69 

wires  passing  through,  and  being  embedded  in  it,  and  the  brass  pins 
for  holding  the  lamp  in  the  lamp-holder  being  also  embedded  in  it. 
For  central  contact  lamps,  the  "vitrite"  has  a  brass  cylinder  out- 
side, to  which  one  end  of  the  filament  is  connected  through  its 
platinum  wire. 

The  light  given  by  the  carbon  filament  depends  upon  its  length 
and  its  surface,  and  the  temperature  to  which  it  is  raised.  What  are 
known  as  high  efficiency  lamps  are  raised  to  a  higher  temperature 
than  the  low  efficiency  lamps,  a  more  powerful  current  being  allowed 
to  pass  through  them. 

The  small  carbon  filament  lamps  are  made  for  5,  8,  12,  16,  25,  32, 
and  50  C.P.  The  16,  25,  32,  and  50  C.P.  lamps  are  all  made  for 
pressures  from  60  to  120,  and  from  150  to  250  volts.  As  the  light 
given  by  a  carbon  filament  depends  upon  its  length,  and  surface,  and 
temperature,  it  will  be  understood  that  a  filament  that  gives  8  C.P. 
with  50  volts,  if  its  length  is  doubled  will  give,  approximately,  16 
C.P.  with  100  volts  ;  and  if  its  length  can  again  be  doubled,  it  will 
give  32  C.P.,  approximately,  with  200  volts.  The  length,  however, 
of  the  filament  of  a  lamp  giving  16  C.P.,  say,  with  200  volts,  is  more 
than  twice  the  length  of  the  filament  of  a  lamp  giving  the  same 
candle-power  with  100  volts,  the  reason  being  the  smaller  surface  of 
the  smaller  filament,  per  inch.  In  order  that  the  filament  may  give 
a  certain  light  with  double  the  pressure,  a  longer  and  thinner  filament 
is  employed,  having  a  higher  resistance,  taking  a  smaller  current,  but 
the  length  is  more  than  doubled  for  double  the  pressure.  Hence,  for 
this  and  for  other  reasons,  the  high  voltage  small  carbon  lamps,  as 
they  are  called,  those  burning  at  from  200  volts,  and  upwards,  are 
not  so  efficient ;  they  require  a  larger  expenditure  of  electrical  energy 
per  C.P.  than  the  lower  voltage  lamps.  A  common  standard  16  C.P. 
lamp,  with  100  volts,  takes  60  watts.  The  16  C.P.  lamp,  to  burn 
with  200  volts,  takes  70  watts.  The  life  of  the  higher  voltage  lamps 
is  also  less  than  that  of  the  lower  voltage.  About  a  mine  there  is  no 
advantage  whatever  in  using  high  voltage  lamps,  except  where  current 
is  taken  directly  from  the  power  service,  as  where  two  220  volt  lamps 
are  connected  across  a  440  volt  service,  or  two  250  volt  lamps  across 
a  500  volt  service.  Wherever  it  can  be  arranged,  it  will  be  found 
more  economical  in  every  way  to  run  the  lighting  service  at  the 
lower  voltage.  The  small  carbon  incandescent  lamps  are  made  to 
burn  at  2J  watts  per  candle,  3  watts  per  candle,  3J  watts  per  candle, 
3£  and  4  watts.  With  a  100  volt  service  a  16  candle  lamp,  at  2J 
watts  per  candle,  takes  O4  of  an  ampere ;  with  an  efficiency  of 
3  watts  it  takes  0'48  amp. ;  with  4J  watts  it  takes  0'56,  and  with 
3|,  the  usual  standard  lamp,  0'6.  Higher  voltage  lamps  and  higher 
candle  lamps  take  current  in  the  same  proportion.  Thus,  the  200 
volt  lamp,  burning  at  2J  watts  efficiency,  takes  0'2  amp. ;  at  3  watts 


70  ELECTRICITY  IN   MINING 

it  takes  0'24  amp.,  and  so  on.  The  100  volt  32  C.P.  3  watts 
efficiency  lamp  takes  0'96  amp.,  and  the  200  volt  lamp  burning 
at  a  similar  efficiency,  0*48,  and  so  on. 

The  Gem  Carbon  Filament  Incandescent  Lamp 

There  is  another  newly  developed  incandescent  lamp,  which  it  is 
hoped  will  compete  with  the  metallic  filament  lamps.  It  has  been 
worked  out  by  the  General  Electric  Co.  of  America,  and,  the  author 
understands,  is  constructed  from  the  ordinary  carbon  filament  by 
subjecting  the  filament  to  a  very  high  temperature  in  an  electrical 
furnace  of  the  resistance  type,  the  furnace  in  which  heat  is  im- 
parted by  raising  the  temperature  of  some  low  conducting  body, 
either  surrounding  or  in  the  furnace,  but  without  any  arc  being 
formed.  It  is  called  a  metallized  filament,  and  it  has  the  property, 
also  possessed  by  metallic  filament  lamps,  that  its  resistance 
increases  with  increasing  temperature,  instead  of  falling  as  in  the 
ordinary  carbon  filament  lamp.  This  change  is  a  most  important 
one,  and  tends  to  lessen  the  winking  of  incandescent  lamps  with 
changes  of  pressure.  With  the  ordinary  carbon  filament  lamp,  when 
the  pressure  increases,  the  current  passing  through  the  lamp  increases, 
and  this  lowering  of  the  resistance  causes  a  further  increase  of  current, 
with  a  further  increase  of  temperature,  and  so  on,  till  the  limit  is 
reached,  the  result  being  that  comparatively  small  increases  and 
decreases  in  pressure  are  visible.  This  has  led  also  to  the  necessity 
of  alternating  currents  being  worked  at  the  comparatively  high 
frequency  of  25  periods  per  second.  Below  25  periods,  and  even 
with  25,  with  some  very  thin  filament  lamps,  there  is  a  distinct 
difference  in  the  light  given  by  the  lamp  at  different  parts  of  the  cycle. 
A  wink  is  distinctly  noticeable.  On  the  other  hand,  for  efficient 
power  distribution,  the  frequency  should  be  as  low  as  possible.  The 
efficiency  of  the  Gem  filament  lamp  is  given  as  2 \  watts  per  candle, 
the  life  being  then  the  same  as  that  of  the  3  watts  per  candle 
ordinary  carbon  filament  lamp.  The  efficiency  and  the  life  will 
probably  be  improved  as  time  goes  on. 

Metallic  Filament  Lamps 

There  are  two  metallic  filament  lamps  on  the  market,  and  others 
are  coming  apparently  very  fast.  Those  on  the  market  are  made  of 
very  fine  wires  of  the  rare  metals,  tantalum  and  osmium.  The  fila- 
ments, as  they  are  termed,  are  very  much  longer  in  the  tantalum  lamp 
than  in  the  carbon  filament  lamp,  and  they  are  formed  into  a  kind  of 
cat's  cradle  inside  the  lamp  globe,  which  is  of  the  same  form  as  that 
of  the  small  carbon  filament  lamp,  the  wire  being  wound  up  and 


ELECTRIC   LIGHTING   FOR   MINES  71 

down  round  glass  hooks  on  a  glass  bridge  fixed  in  the  middle  of  the 
lamp.  Platinum  wires,  as  before,  are  employed  for  connecting  the 
ends  of  the  metallic  filament  lamps  with  the  plates  on  the  lamp  cap, 
which  is  of  the  same  form  as  used  with  the  carbon  filament  lamp. 
The  lamps  are  only  made  at  present  for  pressures  of  50  to  125  volts, 
and  C.P.'s  of  6J  to  26,  so  that  the  lamp  must  be  connected  in 
series  on  a  220  volt  service,  unless  some  arrangement  is  made  for 
supplying  current  at  a  pressure  suitable  for  these  lamps,  as  by  a 
motor  generator.  The  tantalum  lamp  is  made  for  efficiencies  of  from 
1*7  and  2'2  watts  per  candle ;  this  meaning  that  approximately  half 
the  current  is  required  with  any  given  lamps.  The  life  of  the 
tantalum  lamp  is  claimed  to  be  1000  hours,  and  600  hours  before 
the  light  is  reduced  by  20  per  cent.  As  usual  with  a  new  apparatus, 
the  metallic  filament  lamps  are  more  expensive  to  purchase  than  the 
carbon  filament,  but  as  they  consume  so  much  less  current,  the 
difference  is  soon  paid.  The  success  of  the  tantalum  and  osmium 
lamps  has  led  to  other  metals — wolfram,  tungsten,  and  others — being 
employed  for  the  same  purpose. 

The  osmium  lamp  has  developed  into  one  called  the  "  Osram," 
which  is  made  for  C.P.'s  of  0*4  up  to  16,  and  in  the  case  of  lamps  of 
8  C.P.  upwards,  is  made  for  pressures  from  8  volts  upwards.  The 
efficiency  claimed  is  1  watt  per  candle. 


High  C.P.  Incandescent  Lamps 

The  high  C.P.  incandescent  lamp  is  similar  in  construction  to 
the  small  carbon  incandescent  lamp,  except  that  its  filament,  to  use 
the  term  usually  employed,  is  a  stick  of  carbon,  generally  bent  into 
a  U  form.  Its  ends  are  connected  to  platinum  wires,  as  the  small 
carbon  filament  lamps  are,  but  there  are  several  platinum  wires  to 
each  carbon  end,  the  number  depending  upon  the  size  of  the  filament, 
and  the  current  it  is  to  accommodate.  The  filament  is  enclosed  in 
a  globe  of  varying  size,  according  to  the  light  given. 

There  are  two  distinct  forms  of  high  C.P.  incandescent  lamps. 
Those  made  by  The  Sunbeam  Co.,  of  which  the  efficiency  is  2  and  2J 
watts  per  candle,  and  those  made  by  other  firms,  of  which  the 
efficiency  is  more  nearly  4  watts  per  candle.  The  2  watt  efficiency 
lamp  is,  in  the  author's  experience,  a  very  economical  lamp,  and  in 
many  cases  would  be  preferable  to  arc  lamps. 

The  Efficiency  and  Life  of  Incandescent  Lamps. — The  ques- 
tion of  the  efficiency  at  which  carbon  incandescent  lamps  should  be 
run  is  one  that  depends  upon  the  cost  of  the  current.  Where  current 
is  dear,  as  in  some  town  supply  services,  it  is  cheaper  to  burn  lamps 
at  high  efficiency,  and  to  replace  them  when  they  begin  to  give  less 


72  ELECTRICITY  IN   MINING 

light.  Where  the  current  is  cheaper,  as  will  be  the  case  in  the  great 
majority  of  mines  generating  their  own  current,  it  is  more  economical 
in  every  way  to  burn  lower  efficiency  lamps,  and  in  the  author's 
experience  it  has  been  found  better  to  burn  even  low  efficiency  lamps 
at  something  below  their  marked  pressure.  From  the  moment  that 
the  lamp  is  put  into  service,  a  disintegrating  action  commences. 
The  carbon  filament  is  gradually  broken  up,  and  a  bombardment  of 
very  minute  carbon  particles  takes  place,  from  the  filament  on  to  the 
glass,  the  result  being  that  the  filament  takes  less  current,  emits  less 
light,  and  the  glass  gradually  becomes  blackened,  and  transmits  less 
light.  This  action  increases  very  rapidly  with  the  temperature,  and  so 
high  efficiency  lamps  begin  to  blacken  very  much  more  quickly  than 
low  efficiency.  In  addition,  as  explained,  if  the  low  efficiency  lamp 
is  burned  with  a  pressure  five  per  cent,  or  so  below  its  marked  pressure, 
though  it  gives  less  light,  about  twelve  to  fourteen  candles  with  a 
16  C.P.  lamp,  there  is  usually  plenty  of  light  for  a  great  many 
situations,  and  the  life  of  the  lamp  is  very  much  increased.  Only  a 
very  small  percentage  of  the  energy  delivered  to  the  carbon  filament, 
some  three  to  five  per  cent.,  appears  as  light,  even  in  the  very 
high  efficiency  lamps,  the  remainder,  so  far  as  is  known  at  present, 
being  radiated  as  heat. 

Holders  and  Fittings  for  Incandescent  Lamps. — For  small 
carbon  filament  lamps,  the  familiar  bayonet-joint  holder  is  the  almost 
universally  employed  holder.  It  consists  of  a  short  tube  of  stout 
brass,  of  sufficient  length  to  allow- the  lamp-holder  to  enter  nearly  up 
to  the  neck  of  the  lamp.  The  tube  is  slotted  at  opposite  ends  of  a 
diameter,  with  the  straight  and  curved  slots  known  under  the  name 
of  the  bayonet  joint.  The  brass  pins,  which  are  fixed  on  opposite 
ends  of  a  diameter  of  the  cap  of  the  lamp,  are  pushed  into  the  straight 
part  of  the  slot  in  the  holder,  and  when  they  arrive  at  the  curved 
portion,  the  lamp  is  given  a  small  twist  to  the  right,  the  pins  then 
passing  projections  in  the  slots,  and  on  the  lamp  being  released  from 
the  hand,  the  springs  of  the  plungers  described  below,  force  the  pins 
behind  the  projections  in  the  bayonet  slot,  and  lock  the  lamp  in 
position.  At  the  back  of  the  tube  forming  the  lamp-holder,  is  a  disc 
of  highly  glazed  and  special  porcelain,  or  other  material  specially 
arranged  to  withstand  comparatively  high  temperatures.  In  this 
disc  are  fixed  two  little  plunger  contact  pieces.  The  contact  pieces 
are  arranged  to  receive  the  connecting  wires  from  the  supply  service 
through  holes  in  the  porcelain  disc,  the  wires  being  held  in  position 
by  screws  provided  for  the  purpose.  On  each  contact  piece  is  a 
small  hollow  cylinder,  with  a  small  solid  plunger  in  it,  a  small  steel 
spring  being  placed  in  the  tube  behind  the  plunger,  the  result  being 
that  the  plungers  are  forced  outwards,  and  therefore  make  good 
connection  with  the  plates  on  the  bottom  of  the  cap  of  the  lamp, 


ELECTRIC   LIGHTING  FOR   MINES  73 

when  the  latter  is  forced  into  position.  The  slotted  tube  and  the 
porcelain  disc,  and  its  plungers  are  held  in  a  hollow  brass  fitting, 
the  two  portions  being  connected  together  by  brass  rings,  so  that 
the  whole  can  be  taken  apart,  when  the  lamp-holder  is  connected 
to  the  service.  The  different  parts  of  the  lamp-holder  are  shown 
in  Fig.  44.  The  fitting  which  holds  the  tube  and  the  disc,  has 
sometimes  merely  a  female  thread  at  the  end,  sometimes  it  has 
another  screw  with  what  is  termed  a  cord  grip,  two  pieces  of  wood 
or  other  insulating  material  forming  together  a  cylinder,  with 
grooves  for  the  flexible  cord  by  which  the  lamp  is  suspended,  to  lie 
in,  and  arranged  to  be  tightened  up  by  means  of  a  small  ring,  so  as 
to  take  the  weight  of  the  lamp  off  the  connecting  screws  in  the 


FIG.  44. — Showing  the  Different  Parts  of  an  Incandescent  Lamp-holder. 
To  the  Left  is  the  King  with  Female  Thread  which  holds  all 
together.  Next  to  it  is  the  Tube  with  Bayonet  Joint  into  which 
the  Cap  of  the  Lamp  enters.  Next  to  it  is  the  Porcelain  Disc  with 
the  Plunger  Contacts,  and  on  the  Bight  the  Piece  forming  the 
Back  of  the  Holder  through  which  the  Wires  enter. 

plunger  contacts.  Fig.  45  shows  different  forms  of  lamp-holders. 
The  lamp-holder  and  its  lamp  may  be  held  by  merely  screwing 
the  lamp-holder  on  the  end  of  a  piece  of  brass  or  iron  tube, 
forming  a  bracket,  the  other  end  of  the  brass  or  iron  tube  either 
having  a  brass  back  for  fixing  against  a  wall,  or  being  screwed  into 
a  boss  forming  part  of  a  fitting,  to  which  two  or  more  other  tubes 
are  attached.  The  brass  or  iron  bracket  may  also  carry  a  gallery,  on 
which  a  shade  is  held,  very  much  as  in  the  case  of  a  gas  fitting. 
The  above  arrangement  is  that  common  for  houses,  offices,  etc.,  and 
is  suitable  for  the  offices  about  a  colliery,  for  engine-houses  on  the 
surface,  for  weigh-cabins,  and  many  other  places  where  there  is  no 
danger  of  gas,  or  of  the  lamp  being  broken  by  objects  being  thrown, 
or  flying  against  it.  For  a  large  portion  of  mining  work,  however,  for 
the  pit  top,  the  screens,  the  pit  bottoms,  engine-houses  underground, 
roads  underground,  pump-houses,  etc.,  some  protection  is  necessary 


74 


ELECTRICITY  IN   MINING 


for  the  lamp  and  for  the  connections,  both  from  gas  and  from  moisture. 
There  are  many  different  arrangements,  but  they  are  all  on  the  same 
lines.  Usually  for  mining  work  the  wires  which  bring  the  current  to 


FIG.  45. — Forms  of  Lamp-holders.  The  Upper  One  on  the  Left  is  for 
screwing  to  a  Bracket,  and  has  a  Eing  to  hold  a  Conical,  Shade.  That 
on  its  Bight  is  for  Pendant  Lamps.  It  has  the  Cord  Grip  described,  and 
a  Ring  for  holding  a  Shade.  That  on  the  Left  below  is  for  screwing 
against  a  Beam  or  Wall.  It  has  a  Ring  for  holding  a  Shade.  The 
Holder  on  its  Right  is  for  screwing  to  a  Bracket  without  a  Shade- 
carrying  Ring. 

the  lamp  or  lamps,  are  enclosed  inside  a  substantial  iron  tube ;  the 
lamp  end  of  the  tube  has  a  reflector  screwed  to  it,  and  to  the  reflector 
some  arrangement  is  added  that  will  hold  a  glass  shade.  In  addition 


ELECTRIC   LIGHTING   FOR   MINES  75 

a  wire  guard  is  also  sometimes  fixed  outside  of  the  glass  shade.  The 
end  of  the  iron  tube,  away  from  the  lamp,  has  a  flanged  back,  or  some 
arrangement  enabling  it  to  be  fixed  against  a  wall,  a  beam,  prop,  or 
wherever  it  may  be  required.  In  fitting  up,  the  connecting  wires  are 
brought  to  the  lamp-holder,  the  lamp-holder  is  screwed  into  the  end  of 
the  iron  tube,  the  reflector  being  in  its  place,  the  bracket,  or  whatever 
form  the  fitting  may  have  taken,  is  fixed  to  the  wall,  or  wherever  it 
is  to  stand,  a  substantial  hard  wood  block  being  employed  for  the 
purpose,  grooved  to  take  the  wires  leading  from  the  supply  service  to 
the  lamp.  The  lamp  is  then  pushed  into  its  place  in  the  lamp-holder, 
the  glass  shade  with  its  wire  guard  is  put  into  position,  the  latter 
being  held  sometimes  by  a  bayonet-joint  arrangement,  and  sometimes 
by  screws.  In  some  forms  of  fittings  the  reflector  and  the  tube  are 
cast  in  one,  this  making  a  more  watertight  and  gastight  joint,  but 
with  careful  fitting  this  should  not  be  necessary,  and  by  having  the 
reflector  separate,  enamelled  iron,  which  answers  remarkably  well,  can 
be  employed,  and  the  size  of  the  reflector  can  be  made  such  as  will 
be  convenient.  For  a  great  many  situations  the  ordinary  enamelled 
steel  reflector,  blue  on  the  outside,  and  white  on  the  inside,  held  by 
means  of  a  screw  ring  on  the  lamp-holder  will  answer  very  well. 
Places  where  there  is  no  damp,  and  where  there  is  no  danger  of  gas, 
can  be  supplied  in  this  way.  The  enclosed  fitting  with  the  glass 
shade  and  its  wire  guard  make  a  much  better  protection  for  the  lamp 
where  there  is  danger  from  mechanical  injury,  but  it  has  the  very 
serious  disadvantage  that  coal-dust  invariably  works  its  way  inside 
the  protecting  glass,  depositing  sometimes  on  the  lamp  globe  and 
sometimes  on  the  inner  surface  of  the  shade,  but  in  all  cases  obscuring 
the  light,  and  very  often  to  a  serious  extent,  with  the  result  that 
the  fittings  have  to  be  frequently  taken  to  pieces  for  the  purpose  of 
cleaning  the  lamps,  and  fittings  of  this  kind  do  not  last  as  well 
when  they  are  frequently  taken  to  pieces.  For  some  positions,  as 
in  confined  pit  bottoms,  some  pit  heaps,  some  junctions  of  roads,  a 
very  useful  fitting  is  one  that  is  variously  known  as  an  oyster  fitting, 
and  a  bulk-head  fitting,  the  latter  name  being  taken  from  the  fact 
that  it  was  designed  principally  for  use  on  board  ship.  This  fitting 
consists  sometimes  of  a  perfectly  flat  disc,  either  enamelled  or 
painted  white,  arranged  with  a  lamp-holder  inside,  the  connections 
being  brought  to  the  holder,  either  through  a  hole  at  the  back,  or  at 
the  side,  and  the  lamp  being  arranged  to  stand  as  near  the  centre  of 
the  disc  as  possible.  Sometimes  the  disc  is  recessed  or  "  dished,"  as 
it  is  called,  the  object  being  to  give  more  room  for  the  lamp,  and  to 
better  utilize  the  back  rays  from  the  lamp  filament.  In  either  case 
the  fitting  lies  close  to  the  wall,  and  may  be  partially  embedded  in  it. 
It  may  be  fixed  in  a  recess  in  a  wooden  headstock,  where  these  are 
in  use.  Outside  the  lamp,  and  held  by  a  ring  at  the  circumference 


76 


ELECTRICITY  IN   MINING 


of  the  disc,  is  a  semi-cylindrical  glass  globe,  or  one  forming  a  smaller 
portion  of  a  sphere,  the  glass  being  protected  in  some  cases  by  guard 
wires  crossing  it,  and  held  by  the  ring  which  holds  the  glass  in  position. 
Fittings  for  High  C.P.  Incandescent  Lamps.— The  high  C.P. 
incandescent  lamp  is  treated  very  much  in  the  same  manner  as  the 
small  incandescent  lamp  with  reference  to  brackets,  shades,  guards, 
etc.,  except  that  the  tubes  carrying  the  wires,  the  reflectors,  the 
shades,  and  the  guards  must  be  larger  in  proportion.  The  lamp- 
holders,  however,  have  to  be  of  a  different  design.  The  lamp  is  made 
with  a  recess  below  the  neck,  and  the  holder  most  commonly 
employed  consists  of  two  porcelain  discs  held  together  by  bolts  and 
by  the  lamp  terminals,  and  carrying  three  substantial  brass  springs 
which  clasp  the  neck  of  the  lamp,  and  are  held  to  it  by  the  spiral 
spring  shown.  The  platinum  wires  forming  the  terminals  of  the 
filament  are  brought  to  clips  held  under  the  lower  of  the  two  discs, 
to  which  they  are  connected,  and  the  lamp  and  holder  are  suspended 
by  any  convenient  attachment,  held  by  the  porcelain  discs. 

The  Nernst  Lamp 

The  Nernst  lamp,   as   explained,   occupies   a   position   midway 
between   the   arc  and   the  incandescent  lamp.      It  burns   in    the 
atmosphere,  very  much  as   the  arc  does,  merely 
protected  from  the  weather  by  an  outer  globe,  for 
the  same  reason  as  the  arc.    The  light,  however,  is 
given  out  by  a  glowing  pellet  of  the  substances 


FIG.  46. — "  Glower  "  and 
Heating-coil  of  Nernst 
Lamp,  with  the  Porce- 
lain Supporting  Disc. 
The  "Glower"  is  the 
Straight  Portion  inside 
the  Coil. 


FIG.  47.— Another  Form  of  Heat- 
ing-coil for  Nernst  Lamp.  It 
is  seen  above  the  "  Glower." 


FIG.  48.— One  Form 
of  Resistance  used 
with  Nernst  Lamp . 
The  Thin  Iron 
Wire  is  enclosed 
in  a  Small  Tube 
similar  to  a  Small 
Lamp. 


employed  in  the  Welsbach  incandescent  gas  mantle,  made  from  the  very 
refractory  rare  earths — zirconium,  yttrium,  erbium,  and  others.     The 


ELECTRIC  LIGHTING   FOR   MINES 


77 


pellet  is  called  a  "  glower."  It  will  not  glow,  however,  and  it  will 
not  allow  a  current  to  pass  that  will  cause  it  to  glow,  unless  it  is 
first  raised  to  a  certain  temperature.  When  cold,  the  glower  has 
such  a  high  resistance  that  practically  no  appreciable  current  passes 
through  it.  When  raised  to  a  certain  critical  temperature,  current 


FIG.  49.— "A"  Type  Nernst  Lamp  with 
"Glower"  vertical.  The  Heating- 
coil  is  seen  surrounding  the  "  Glower, ' ' 
The  Lamp-holder  is  above  the  Lamp. 
The  Terminals  are  seen  on  the  Out- 
side. 


FIG.  50. — "  B  "  Form  of  Nernst  Lamp 
with  Horizontal  "  Glower "  and 
Heating-coil  and  Globe.  The  Holder, 
with  the  Electro-magnetic  Cutout 
and  Resistance,  are  inside  the  Metal 
Case  forming  the  Upper  Part  of  the 
Apparatus. 


commences  to  pass  through  it  in  an  appreciable  quantity,  raising  its 
temperature,  and  causing  it  to  give  out  an  intensely  bright  white  light. 
The  heat  required  is  delivered  to  the  glower  by  a  small  coil  of 
platinum  wire,  through  which  the  current  is  passed  when  the  lamp  is 


78  ELECTRICITY   IN   MINING 

first  switched  on,  as  shown  in'  Figs.  46  and  47,  as  well  as  passing 
through  the  glower  itself.  The  glower  takes  from  -|  to  1|  minute 
to  acquire  the  necessary  temperature,  and  when  the  lamp  is  burning 
normally,  the  platinum  heating  coil  is  switched  out  by  a  small 
electro-magnetic  switch,  which  then  comes  into  operation.  In 
addition  to  the  above,  it  is  necessary  to  have  a  resistance  in  circuit 

with  the  glower,  very  much  in  the 
B  same  way  as  a  resistance  is  fixed  in 
circuit  with  an  arc  lamp,  though  for 
not  quite  the  same  reason.  If  the 
pressure  at  the  terminals  of  the  glower 
could  be  depended  upon  not  to  vary 
under  any  circumstances,  the  resistance 
would  not  be  required ;  but  as  this  is 
often  difficult  to  arrange,  and  is  never 
attained  in  the  ordinary  town  supply 
service,  for  which  the  Nernst  lamp  has 
been  principally  worked  out,  a  small 
piece  of  thin  iron  wire,  contained  in  a 
small  glass  tube  from  which  the  air 
has  largely  been  exhausted,  is  inserted 
in  the  circuit.  It  has  a  very  critical 
temperature,  just  as  the  glower  pellet 
has.  If  the  iron  wire,  when  at  this 
temperature,  is  subject  to  further  heat- 
ing, its  resistance  rises  very  consider- 
ably for  a  comparatively  small  increase 
of  heat,  and  therefore  for  a  com- 
paratively small  increase  of  current. 
On  the  other  hand,  if  it  is  deprived  of 
a  small  quantity  of  heat,  and  therefore 
of  current  at  this  temperature,  its 

FIG.  jii.— Connections  ^  of  ^Nernst  resistance    falls     considerably.       The 

of  this  is,  if  the 


. 

is  broken  when  the  "  Glower "  is  supply  increases,  the  increased  resist- 
heated,  O  is  the  "Glower,"  H  ance  of  the  iron  wire,  the  compensating 
^eWAf  ttnSg:  resistance,  «  it  is  called,  uses  up  thS 
coil  out  of  Circuit,  R  the  Balancing  increased  pressure,  the  pressure  at 
Eesistance.  the  terminals  of  the  glower  remain- 

ing  practically   constant,   and   if  the 

pressure  falls,  the  lower  resistance  of  the  iron  wire  compensator 
frees  a  certain  pressure,  which  again  maintains  the  pressure  at  the 
terminals  of  the  glower  practically  constant.  The  resistance  is 
shown  in  Fig.  48.  The  whole  of  the  above  apparatus  is  contained 
in  the  lamp-holders  shown  in  Figs.  49  and  50,  to  which  the  wires 


ELECTRIC   LIGHTING  FOR   MINES 


79 


connected   to   the  supply  service   are  brought,  and  which   also  is 

arranged   to  carry  a  globe 

or  shade,   as  may  be   re- 

quired.     The    connections 

of  the  lamp  are  shown  in 

Fig.  51.     The  Nernst  lamp 

is  claimed   to   work  with 

an  efficiency  of  1*5   watts 

per  candle,  and  it  should 

be   of  service  for   engine- 

houses,     pit     banks,     pit 

bottoms   with   high   roofs, 

and     many    other    places 

where    a    good    light    is 

required.     Fig.    52   shows 

the  distribution  of  the  light 

with  different  globes.    The 

lamp  is  made  for  both  con- 

tinuous    and     alternating 

currents;   but  those  made 

for     alternating      currents   FIG.  52.  —  Curves   showing  the   Distribution   of 

rrmcif-  not   V»P  nqprl  fnr  rnn          Light  with  Nernst  Lamp    when  enclosed    in 

must  not  be  used  lor  con-  Pofnted  and  Round  Hoiophone  Globes.  The 
tmuous,  and  Vice  versa.  Full  Curve  is  with  the  Pointed  Globe,  and  the 
Further,  the  lamp  must  Dotted  Curve  with  the  Bound.  It  will  be  noticed 

that  the  LiSht  is  more  distributed  with  the 

Pointed  than  the  other>  The  Lengths  of  the 
Lines  meeting  the  Curves  give  the  Proportional 
intensity  of  Light  at  the  Different  Points. 


as  shown,  the  plus  Wire  of 


a  continuous  current  service 

being     connected     to    the 

terminal  of  the  lamp  marked   plus.     The  reason  of  this  is,  there 

is  apparently  a  transference   of  material  from  the  positive   to   the 

negative  terminal  of  the  glower. 


Delivering  the  Current  to  Incandescent 
and  Nernst  Lamps 

Incandescent  lamps  and  Nernst  lamps  may  be  fixed  anywhere, 
and  may  take  their  current  from  any  cables  where  the  pressure  for 
which  they  are  made  is  available,  with  the  reservation  mentioned  in 
the  case  of  the  Nernst  lamp,  that  only  lamps  made  for  alternating 
currents  are  to  be  used  on  alternating  current  services.  As  explained 
in  Chapter  I.,  the  ordinary  carbon  filament  lamp,  and  the  metallic 
filament  lamp,  are  marked  for  pressures  which  are  applicable  to 
either  continuous  or  alternating  currents,  and  they  will  burn  upon 
any  circuit,  continuous,  single,  two  or  three  phase,  upon  which  this 


8o  ELECTRICITY   IN   MINING 

pressure  exists.  Only  single-phase  currents  must,  of  course,  be  used 
for  incandescent  lamps,  but  the  lamps  may  be  connected  in  each 
phase  of  a  two-phase  service,  and  between  each  pair  of  the  three 
wires  of  a  three-phase  service.  They  may  also  be  connected  between 
each  of  the  three  wires  of  a  three-phase  service,  and  the  wire  con- 
nected to  the  neutral  point  of  the  service  with  star  connection,  as 
will  be  explained,  always  providing  that  the  pressures  are  those  for 
which  the  lamps  are  made.  Incandescent  lamps  may  be  run  two  or 
more  in  series  where  convenient,  as  two  220  volt  or  two  250  volt 
lamps  in  series  between  the  cables  of  a  440  or  500  volt  service,  no 
matter  whether  the  pressure  is  that  of  a  continuous  current  power 
service,  or  a  two  or  three  phase  service.  It  is,  however,  unwise  to 
run  incandescent  lamps  in  series  as  a  general  thing.  Near  the  face 
where  only  a  440  or  500  volt  service  exists,  the  inconvenience  to  be 
mentioned,  is  more  than  outweighed  by  the  convenience  of  obtaining 
a  supply  of  current  for  light  at  the  face.  The  inconvenience  is,  it  is 
found,  that  when  incandescent  lamps  are  run  in  series,  leakage 
occurs  at  each  connection  between  the  lamps,  with  the  result  that 
the  positive  lamp  of  the  series  receives  more  current  than  it  should 
do,  while  each  successive  lamp  receives  less  and  less  current,  and 
gives  less  and  less  light,  the  positive  lamp  giving  a  much  brighter 
light  than  the  others,  and  burning  out  much  more  quickly.  There  is 
against  this  the  solid  advantage,  at  a  pit  bottom,  for  instance,  of 
making  only  one  break  in  the  insulation  of  the  cable  for  several 
lamps.  The  author's  view  is,  that  incandescent  lamps  at  the  pit 
bottom,  in  large  engine-houses  underground,  and  almost  everywhere 
where  a  certain  number  of  lamps  are  required,  will  be  economically 
and  conveniently  run  from  motor  generators,  generating  current  at 
100  or  110  volts.  As  explained,  the  lamps  are  stronger  and  more 
efficient,  and  the  lighting  service  is  kept  quite  independent  of  the 
power  service,  no  matter  what  the  pressure  of  the  latter  may  be, 
while  motor  generators  are  made  of  any  size,  and  can  be  fixed 
practically  anywhere  about  the  pit.  There  is  no  more  difficulty  in 
arranging  a  100  volt  lighting  service  off  a  3000  volt  three-phase 
service  with  a  motor  generator,  than  from  a  500  volt  continuous 
current  service. 

Portable  Electric  Lamps 

From  the  moment  the  incandescent  lamp  became  practicable,  it 
was  the  dream  of  mining  engineers  and  electrical  engineers  to  adapt 
it  for  portable  lamps,  to  take  the  place  of  the  ordinary  Davy  or 
Clanny  lamps.  In  the  very  early  days  an  attempt  was  made  to  use 
lamps  at  the  face  of  the  coal,  the  lamps  being  rendered  portable  by 
connecting  them  to  flexible  wires  leading  to  supply  cables,  but  it  was 


ELECTRIC   LIGHTING   FOR   MINES  81 

soon  seen  that  this  was  impracticable.  There  was  too  much  danger 
of  wires  being  broken  and  causing  sparks  that  might  fire  gas.  "With 
the  development  of  the  flexible  cables  for  coal-cutting  machines, 
portable  lamps  on  these  lines  might  be  arranged  but  for  the  difficulty 
that  the  motors  of  coal-cutting  machines  are  run  at  pressures  of 
500  volts  or  thereabouts,  and  this  necessitates  at  least  two  incan- 
descent lamps  in  series,  with  the  additional  danger  of  breakage  of 
the  cable  and  additional  complication. 

Very  early,  also,  after  the  invention  of  the  incandescent  lamps, 
attempts  were  made  by  Sir  Joseph  Swan  and  others,  to  construct 
small  self-contained  lamps  with  small  batteries  of  accumulators 
carrying  a  small  incandescent  lamp,  either  on  the  side  or  at 
the  top.  The  great  objection  was,  if  they  were  made  sufficiently 
strong,  and  with  a  sufficient  capacity  for  a  working  shift  and 
something  over,  they  were  too  heavy.  The  latest  form  of  Davy 
lamp  weighs  a  little  over  2  Ibs.,  and  lamps  giving  a  very  good  light 
indeed  weigh  only  3J  Ibs.,  while  the  electric  lamps  with  their 
accumulators  weighed  7  or  8  Ibs.  One  lamp,  however,  has  sur- 
vived, known  as  the  Sussman.  In  this  lamp  there  are  two  small 
accumulator  cells  of  the  lead-oxide  type,  held  inside  a  thin  iron 
case,  and  with  a  small  4- volt  lamp  carried  on  the  top  inside  a  glass 
cylinder,  and  held  by  two  porcelain  reflectors,  very  much  on  the 
lines  of  the  ordinary  miner's  lamp.  The  difficulty  of  the  slopping  of 
the  acid  has  been  overcome  by  filling  the  space  between  the  plates 
with  a  substance,  such  as  finely  divided  cork,  that  will  absorb  the 
dilute  acid,  just  as  a  sponge  does,  without  adding  very  much  to  the 
resistance.  The  lamp  gives  about  1  C.P.  when  the  battery  is  first 
charged,  and  it  will  go  on  giving  a  light,  gradually  decreasing,  for 
about  15  hours,  so  that  if  a  man  should  be  left  in  the  pit,  he  has 
several  hours'  light  in  his  lamp.  A  small  switch  is  attached  to 
the  lamp,  which,  in  the  author's  opinion,  is  its  weak  point, 
inasmuch  as  there  is  not  space  to  make  it  sufficiently  strong  for 
mining  work.  On  the  other  hand,  there  is  no  doubt  that  the  use  of 
the  switch  enables  the  capacity  of  the  accumulator  to  be  very 
economically  employed.  Lamps  are  charged,  and  are  got  ready  for 
the  miners,  but  need  not  be  switched  on  until  the  individual  miner 
presents  himself  at  the  lamp  cabin ;  and  therefore  the  storage  battery 
is  only  losing  by  leakage,  and  not  by  actual  current. 

The  lamps  are  arranged  to  be  easily  taken  to  pieces,  the  accumu- 
lators being  charged  apart  from  the  lamps,  and  the  usual  method  is, 
a  number  of  them  are  arranged  in  a  series,  and  connected  across  the 
lighting  service  with  a  lamp  in  series  with  them,  to  show  that  the 
current  is  passing.  In  the  author's  opinion  this  method  is  not  really 
economical.  A  much  better  plan  would  be  to  fix  a  motor  generator 
in  the  lamp  cabin,  the  motor  taking  its  current  from  the  supply 

G 


82  ELECTRICITY   IN   MINING 

service,  whatever  it  may  be,  and  the  generator  furnishing  current  at 
about  6  volts  to  a  pair  of  bus  bars,  between  which  connections 
would  be  made  for  individual  pairs  of  cells,  so  that  the  lampman 
would  merely  have  to  take  the  lamp  apart,  put  the  cells  into  position, 
and  leave  them  to  charge.  There  should  be  some  arrangement  for 
indicating  that  each  battery  was  being  properly  charged,  and  this 
could  either  be  one  of  the  lamp  globes  employed  with  the  lamp  itself, 
or  preferably  a  small  low-reading  ampere  meter,  showing  exactly 
the  current  that  was  going  into  the  battery.  The  portable  electric 
lamp  suffers  still,  however,  from  one  want.  The  ordinary  miner's  lamp 
performs  two  offices.  It  gives  light,  and  it  indicates  the  presence  of 
gas,  the  latter  being  accomplished  by  the  elongation  of  the  flame  when 
gas  is  present,  experienced  miners  being  able  to  tell  by  the  con- 
dition of  the  flame,  approximately  the  percentage  of  gas  present. 
Several  attempts  have  been  made  to  add  gas-detecting  apparatus  to 
the  portable  electric  lamp.  The  Sussmann  Co.  introduced  an 
apparatus  of  the  kind,  which  lighted  a  small  red  lamp  attached  to  the 
lamp-case,  in  the  presence  of  gas.  And  this  was  shown,  and  worked 
apparently  quite  satisfactorily  with  ordinary  town's  gas.  Ordinary 
town's  gas,  however,  consists  very  largely  of  hydrogen,  and  it  was 
shown  that  it  was  the  hydrogen  gas  which  was  operating  the  gas 
indicator.  Mr.  H.  J.  Prested  worked  out  an  apparatus  which  was 
free  from  this  objection,  in  which  the  properties  of  a  porous  diaphragm 
were  made  use  of,  the  gas  in  an  explosive  atmosphere  passing  through 
a  porous  diaphragm  into  a  closed  chamber,  and,  operating  a  contact, 
closing  an  electric  circuit  in  which  was  an  incandescent  lamp 
coloured  red,  connected  to  the  accumulator.  The  author  believes  that 
though  this  was  very  successful,  not  much  has  yet  been  done 
with  it. 


CHAPTER   IV 

THE  GENERATION  OF  ELECTRICITY 
The  Electricity  Generating  Station 

IT  is  now  acknowledged  that  the  most  economical  method  of  distri- 
buting power  about  a  mine,  or  to  any  group  of  mines,  or  in  fact  to  any 
works  occupying  a  large  area  is,  by  generating  electricity  at  some 
convenient  point,  as  central  as  possible,  where  a  plentiful  supply  of 
water  is  available,  and  where  coal  can  be  delivered  cheaply  and 
conveniently,  the  energy  being  transmitted  by  means  of  cables  to  the 
different  points  where  power  is  required,  and  then  employed  to 
drive  motors  of  different  sizes  and  types.  The  same  generating 
station,  but  usually  a  different  set  of  plant  in  the  station,  can  be 
employed  for  generating  current  for  the  lighting  service  all  over  the 
works.  In  order,  however,  that  power  shall  be  distributed  economi- 
cally to  the  different  points  of  consumption,  it  is  necessary  that  the 
electric  current  shall  be  generated  economically.  That  is  to  say,  the 
minimum  quantity  of  fuel,  or,  to  put  it  in  another  way,  the  minimum 
generating  cost  that  is  possible,  must  rule,  and  in  order  to  accomplish 
this,  every  part  of  the  generating  plant  must  be  of  the  very  highest 
possible  efficiency,  and  every  advantage  must  be  taken  of  the  appliances 
now  on  the  market  for  economizing  fuel  consumption.  It  will  be 
understood,  also,  that  any  source  of  power,  wind,  water,  coal,  or  oil 
that  can  be  arranged  to  drive  a  dynamo,  can  be  made  use  of. 

The  Possibility  of  using  Water  Power 

One  of  the  most  important  matters  in  connection  with  economical 
generation  of  electricity  is  the  position  of  the  generating  station. 
For  nearly  all  coal  and  iron  works,  coal  will  naturally  be  employed  as 
fuel,  but  there  will  be  a  few  cases,  very  few  in  the  United  Kingdom, 
where  it  will  pay  to  employ  water  power  that  may  exist  in  the 
neighbourhood,  to  generate  electric  currents  for  lighting  the  mine 

83 


84  ELECTRICITY   IN   MINING 

and  for  the  power  service,  the  whole  of  the  coal  raised  being  sold. 
There  is  at  least  one  case  on  the  Continent  of  Europe  where  this  is 
done,  though  the  conditions  are  very  different  from  anything  that 
exists  in  this  country.  Still,  as  the  author  hopes  that  this  book  may 
be  of  use  to  mining  engineers  in  other  countries,  where  water  power  is 
more  abundant  than  in  this  country,  it  may  be  of  interest  to  discuss 
the  question.  In  every  case  of  this  kind,  in  fact,  in  every  engineering 
problem,  there  is  one  rule  which  should  govern  the  use  of  apparatus 
of  different  kinds,  viz.  the  balance  sheet.  In  the  present  instance, 
even  in  the  most  favourably  situated  collieries,  there  will  be  a  certain 
residue  of  coal  which  can  be  used  to  fire  the  boilers,  unless  it  can  be 
sold  for  more  than  the  power  which  is  to  take  the  place  of  the  steam 
it  would  generate  will  cost.  It  is  often  supposed  that  water  power 
costs  nothing.  Mining  engineers  will  know  very  much  better  than 
this,  because  they  frequently  have  to  pay  high  prices  for  water  for 
their  boilers.  The  reason  for  the  high  price  of  water  for  colliery 
boilers  is  not  exactly  the  same  as  the  reason  that  water  power  costs 
something,  but  part  of  the  reasons  for  one  are  the  reasons  for  the 
other.  Water  power  can  only  be  employed  when  it  can  be  stored  in 
sufficient  quantity,  in  reservoirs  or  ponds,  to  insure  that  there  is  a 
supply  of  water  for  driving  the  water-engines  at  all  seasons  ;  unless 
some  river  can  be  tapped  at  a  high  level,  and  the  water  allowed  to 
run  through  the  water-engine  to  a  lower  level,  as  is  done  so  frequently 
in  those  parts  of  America  where  there  is  no  coal.  In  either  case  there 
is  the  cost  of  making  the  reservoir,  the  canal  or  pipe  line  which  is  to 
lead  the  water  to  the  water-engine,  and  there  is  frequently  the  cost 
of  the  electrical  transmission  line  to  the  point  where  the  power  is 
required.  The  cost,  then,  of  water  power  which  has  to  be  reckoned 
in  the  balance  sheet  is  the  interest  on  the  cost  of  making  the 
reservoir,  the  canal,  pipe  line,  shafts  where  they  are  required,  as  at 
Niagara,  together  with  that  on  the  water  engines  and  power  house  and 
transmission  line.  On  the  other  side  of  the  balance  sheet  would  be 
placed  the  cost  of  fuel  and  water  which  has  to  be  paid  for,  together 
with  the  interest  and  up-keep  of  the  boilers,  engines,  and  accessories, 
or,  as  explained,  the  value  of  the  coal  that  would  have  been  employed 
for  raising  steam.  In  striking  the  balance  sheet  where  water  power  is 
to  be  employed,  when  it  costs  less  than  the  coal  it  is  to  displace,  the 
balance  sheet  will  include  on  the  water  side  the  interest  and  up- 
keep on  the  water  engine  and  accessories,  and  the  interest  on  the 
transmission  line,  less  the  interest  on  the  steam  plant.  If  the  differ- 
ence between  these  is  less  than  the  fuel  displaced  can  be  sold  for,  and 
there  is  a  market  for  it  all  the  year  round,  while  water  power  is  sure 
all  the  year  round,  it  is  economical  to  employ  the  water  power. 
Unless  this  can  be  insured  it  is  not  economical. 


THE   GENERATION   OF   ELECTRICITY  85 


How  the  Water  Power  is  measured 

Before  it  is  decided  to  employ  water  power,  however,  it  is  always 
as  well  to  know  that  there  is  sufficient  energy  in  the  water  available. 
There  is  very  often  a  very  hazy  idea  as  to  what  is  required.  Water 
power  follows  the  same  laws  as  every  other  source  of  energy. 
33,000  Ibs.  of  water  falling  through  1  foot  in  one  minute,  or  any 
equivalent  numbers,  say  33  Ibs.  of  water  falling  through  1000  feet  in 
one  minute,  will  furnish  1  H.P.  A  gallon  of  water  weighs  10  Ibs., 
therefore  100  gallons  of  water  falling  through  33  feet  in  one  minute 
will  furnish  1  H.P.  The  first  thing,  therefore,  to  be  done  is  to 
measure  the  quantity  of  water  available,  and  the  measurement  must 
be  taken  at  the  most  unfavourable  time  of  the  year,  when  the  quantity 
of  water  is  least.  Full  instructions  are  given  for  the  measurement 
of  the  water  passing  in  any  stream,  or  available  at  any  point,  in  the 
books  on  Hydraulics.  The  measurement  is  a  somewhat  troublesome 
process,  requiring  a  certain  amount  of  skill  to  perform  accurately. 
It  depends  upon  the  actual  measurement  of  a  certain  cubical  content 
of  water  passing  over  a  certain  space  in  a  certain  time.  But  this  is 
only  the  commencement  of  the  problem.  Having  obtained  the 
quantity  of  water  that  can  be  depended  upon  at  all  seasons  of  the 
year,  the  weight  of  the  quantity  passing  per  minute  multiplied  by 
the  distance  through  which  it  falls,  gives  the  total  possible  energy 
available  from  that  source.  But  care  must  be  exercised  in  seeing 
that  the  distance  through  which  the  water  falls  is  measured,  or 
estimated  on  practical  lines.  That  is  to  say,  that  distance  only  that 
can  be  used  in  the  water  engines  must  be  taken  as  the  fall.  In  the 
case  of  a  waterfall  such  as  Niagara,  or  the  Victoria  Falls  on  the  Zam- 
besi, where  Nature  has  thrown  a  dam  across  the  rivers  which  those  falls 
intercept,  the  total  height  of  the  fall  cannot  be  used  in  the  calculation, 
because  some  portion  of  the  height  must  be  left  for  the  water  which 
has  passed  through  the  water  engine,  and  that  has  done  its  work,  to 
get  away  freely,  without  forming  eddies,  as  these  may  set  up  back 
pressures  in  the  water  engine  itself.  Similarly,  where  the  river  is 
dammed,  and  a  canal  or  a  pipe  line  is  taken  from  the  side  of  the 
dam  to  some  convenient  point,  and  there  connected  to  the  water 
engine,  the  water  being  allowed,  after  passing  through  the  water 
engine,  to  find  its  way  into  the  river  below,  or  into  some  other  river, 
as  in  the  case  of  the  St.  Lawrence  and  its  tributary  the  Eiver  Grasse, 
the  total  height,  the  total  distance  that  can  be  employed  in  the 
calculation  referred  to  above,  is  the  difference  between  the  vertical 
height  of  the  darn  at  most  unfavourable  seasons  and  that  of  the 
water  in  the  tail  race,  as  the  conduit  or  passage  which  carries  away 
the  water  which  has  done  its  work  in  the  engine  is  called.  And  there 


86  ELECTRICITY  IN   MINING 

is  another  thing  to  be  looked  out  for  in  this  matter.  In  times  of 
drought  the  water  will  be  low  in  the  reservoir,  or  the  upper  part  of 
the  river  from  which  it  is  taken,  and  the  available  fall  will  be  smaller 
from  this  cause  than  when  the  river  is  full.  On  the  other  hand,  in 
times  of  flood,  while  the  rise  of  the  river  at  first  will  increase  the 
available  fall ;  after  a  certain  time,  if  the  flood  goes  on,  the  water  in 
the  lower  part  of  the  river  and  in  the  tail  race  also  rises,  and  the 
available  fall  is  again  reduced  from  that  cause.  Having,  however, 
obtained  the  available  quantity  of  water  passing  per  minute  under 
the  most  unfavourable  conditions,  and  the  available  height  through 
which  its  fall  can  be  taken,  also  under  the  most  unfavourable  con- 
ditions, the  power  obtainable  is  a  certain  fraction  of  this,  depending 
upon  the  efficiency  of  the  water  engine.  The  efficiencies  of  water 
engines  vary  from  40  per  cent,  to  87  per  cent.  In  addition  to  this, 
there  will  be  the  charge  made  by  the  electrical  generators  for  con- 
verting the  mechanical  power  of  the  water  engine  into  the  electrical 
power ;  there  will  be  the  charge  made  by  the  electrical  conductors  for 
transmitting  the  power  from  the  place  where  it  is  generated  to  the 
place  where  it  is  to  be  consumed,  say  to  the  mine ;  and  in  nearly 
every  case  there  will  also  be  a  charge  at  the  point  of  consumption, 
made  by  some  apparatus  for  converting  the  current  from  a  high  to 
lower  pressure.  In  the  great  majority  of  cases  it  will  be  necessary  to 
generate  current  at  the  water-power  station  at  a  comparatively  high 
pressure,  in  many  cases  to  transform  it  up  to  a  very  high  pressure, 
10,000  volts  or  more,  in  order  to  keep  the  size  of  the  transmission 
conductors  down,  and  it  will  then  be  necessary  to  retransform  the 
high-pressure  current  to  lower  pressures  for  use  at  the  mines,  etc. 


Forms  of  Water  Engines 

The  author  does  not  propose  to  enter  very  much  into  the  details 
of  water  engines  in  this  book.  As  explained,  the  water  must  be 
stored  unless  Nature  has  done  the  work,  as  in  the  great  lakes  above 
Niagara,  or  in  the  great  rivers  from  which  power  is  taken  in  America, 
India,  and  elsewhere.  The  water  must  be  taken  by  iron  or  wood 
pipes,  troughs,  conduits,  canals,  or  any  other  convenient  method  to 
the  water  engine,  and  provision  must  be  made  for  delivering  the  water 
to  the  engines,  for  regulating  the  quantity  passing  into  each  particular 
engine,  and  for  allowing  the  water  to  run  away  freely  after  it  has 
done  its  work.  There  are  practically  three  forms  of  water  engine  : 
the  water  wheel  proper,  which  was  so  much  used  by  the  sides  of  the 
old  mill  streams  in  the  United  Kingdom  before  the  advent  of  coal ; 
the  turbine,  which  came  into  being  for  the  purpose  of  utilizing  small 
quantities  of  water  falling  from  great  heights,  as  in  Switzerland  and 


THE   GENERATION   OF  ELECTRICITY  87 

elsewhere;  and  the  water  engines  of  the  type  of  the  Pelton  water 
wheel,  which  may  be  considered  as  something  between  the  large 
water  wheel  and  the  turbine.  The  large  water  wheel  and  the  water 
turbine  correspond  roughly  to  the  slow  speed  and  high  speed  steam 
engines.  The  large  water  wheel  has  usually  a  very  large  diameter,  and 
it  revolves  at  a  very  slow  rate,  sometimes  only  a  few  revolutions  per 
minute.  Power  is  obtained  in  one  form  of  wheel  by  allowing  the 
water  which  has  been  brought  down  by  the  pipe  line,  or  canal,  to  fill 
buckets  which  are  carried  at  intervals  on  the  periphery  of  the  wheel. 
The  weight  of  the  water  in  the  bucket  falling  through  a  vertical 
height,  corresponding  to  a  portion  of  the  diameter  of  the  wheel, 
delivers  the  energy  liberated  by  its  fall  to  the  periphery  of  the 
wheel,  causing  the  wheel  itself  to  revolve  in  the  process.  As  there 
is  always  a  bucket  coming  under  the  mouth  of  the  pipe  line,  and  one 
or  more  buckets  always  falling  from  that  position  to  the  bottom  of 
the  wheel  pit,  the  motion  is  continuous,  and  the  useful  energy  the 
wheel  is  able  to  deliver  is  made  up  of  the  sum  of  the  energies 
delivered  by  the  active  buckets  during  any  revolution,  less  the 
charge  made  by  friction,  and  by  the  eddies  in  the  tail  race.  In 
another  form  of  wheel,  the  pressure  of  the  water,  that  is  to  say,  the 
weight  of  the  water  column  above  that  which  is  actually  impinging 
on  the  wheel,  is  made  to  act  upon  successive  projections  of  the  wheel 
and  to  force  it  round.  As  there  is  always  one  of  these  projections, 
either  actually  in  front  of  the  stream  of  water  or  coming  to  it,  the 
action  of  the  water  upon  the  wheel  is  continuous,  and  the  motion  of 
the  wheel  itself  is  continuous.  The  energy  available  at  the  wheel 
shaft  is  that  of  the  pressure  of  the  water  column,  that  is  to  say,  the 
weight  of  the  column  of  water  above  the  wheel,  multiplied  into 
the  actual  quantity  of  water  passing  per  minute,  less  any  water  that 
may  be  wasted,  less  also  the  friction  of  the  wheel  and  any  back 
pressure  created  by  eddies.  In  the  turbines,  of  which  there  are 
various  forms,  there  are  always  a  number  of  curved  blades  surround- 
ing a  central  shaft,  and  the  column  of  water  is  made  to  impinge 
upon  each  of  the  blades  in  succession.  The  water  sometimes  flows 
outwards  from  the  centre,  and  sometimes  inwards,  the  latter  giving 
the  highest  efficiency.  A  portion  of  the  force  of  the  water  acts  in  the 
direction  of  rotation  of  the  shaft,  and  as  there  is  always  a  blade 
receiving  the  force  of  the  water,  the  motion  is  continuous,  and  the 
energy  available  at  the  shaft  of  the  turbine  is  that  portion  of  the 
energy  of  the  water  which  is  resolved  in  the  direction  of  the  rotation 
of  the  shaft,  less  the  friction  of  the  shaft  and  of  the  blades,  and  less 
any  back  pressure  created  by  eddies,  etc.  This  is  hardly  the  place 
to  go  into  the  question  of  the  construction  of  turbines,  but  it  may 
be  mentioned  that  the  efficiency  of  the  turbines  depends  very  much 
upon  the  form  of  the  blades.  What  is  required  is,  that  the  blade 


88  ELECTRICITY  IN   MINING 

shall  be  of  such  a  form  as  to  convert  the  largest  possible  portion  of 
the  force  delivered  to  it  into  rotary  motion  of  its  shaft,  and  to  deliver 
the  water  which  has  done  its  work  upon  the  blades  in  such  a 
condition  that,  on  the  one  hand,  as  much  of  the  available  energy  as 
possible  has  been  taken  out  of  it  in  passing  through  the  turbine,  and, 
on  the  other  hand,  that  the  water  escaping  from  the  apparatus  shall 
not  be  able  to  set  up  eddy  currents  which  would  reduce  the  efficiency 
of  the  apparatus. 

Short  Description  of  Pelton  Wheel 

In  the  Pelton  water  wheel  there  are  a  number  of  buckets, 
arranged  round  the  periphery  of  the  wheel  in  the  usual  way,  but  the 
buckets  are  of  a  peculiar  form.  Practically,  each  bucket  consists  of 
two,  connected  at  the  middle,  and  the  water  is  delivered  against  the 
buckets  in  the  form  of  jets,  from  nozzles,  something  after  the  manner 
of  the  nozzles  of  the  De  Laval  and  other  steam  turbines.  The  jet  of 
water  divides  between  the  two  halves  of  the  bucket,  and  is  deflected 
to  both  sides  out  of  the  way  of  the  wheel,  the  result  being  a  very  high 
efficiency.  It  is  known  as  the  tangential  wheel.  Any  one  of  the 
water  engines  described  may  be  used  to  drive  electricity  generators, 
either  by  coupling  the  axles  of  the  water  engines  with  the  axles  of 
the  dynamos,  or  by  driving  the  latter  by  belts  or  ropes. 


Steam  Plant 

As  already  mentioned,  coal  will  in  the  great  majority  of  cases  be 
the  source  of  energy  for  lighting  and  power  in  mines.  There  are  two 
methods  of  obtaining  the  energy  of  the  coal — by  burning  it  in  the 
furnace  of  a  boiler  to  generate  steam,  and  by  using  it  to  generate  gas 
in  a  producer.  There  are  two  principal  forms  of  boilers,  known 
respectively  as  the  Water  Tube  and  Fire  Tube,  the  latter  being  more 
generally  known  as  Lancashire,  or  Cornish,  or  marine  type  boilers. 
The  Lancashire  type  of  fire-tube  boilers  is  more  generally  used  than 
the  others.  The  principal  distinction  between  the  two  forms  of 
boiler  is,  in  the  water-tube  boiler  the  water  which  is  to  be  con- 
verted into  steam  circulates  inside  of  tubes  arranged  for  the  purpose, 
connected  with  drums  for  steam,  and  for  water,  as  will  be  explained, 
the  gases  from  the  boiler  furnace  playing  around  the  outside  of  the 
tubes.  In  the  fire-tube  boiler  the  gases  from  the  furnace  are  made 
to  pass  through  tubes  or  flues  while  the  water  circulates  in  the 
containing  vessel  around  them.  In  the  Lancashire  boiler,  which  is 
long  in  comparison  with  its  diameter,  and  cylindrical  in  section, 
there  are  two  tubes  or  flues  extending  the  whole  length  of  the 


PLATE  IA. — Section  of  Babcock  &  Wilcox's  Water  Tube  Boiler,  with  Chain 
Grate  Stoker  and  Superheater.  The  Superheater  is  the  Coil  of  Pipes 
above  the  Boiler  Tubes. 


PLATE  IB. — "Climax"  Boiler  as  made  by  Messrs.  B.  Rowlands, 
at  the  Top  is  a  Portion  of  the  Boiler  Tubes. 


The   Coil 


[To  face  p.  88. 


THE   GENERATION   OF  ELECTRICITY  89 

boiler,  and  occupying  a  large  portion  of  the  available  space,  and  the 
gases  which  are  generated  by  the  combustion  of  the  coal  pass  through 
these  flues  on  their  way  to  the  chimney.  In  Messrs.  Galloway's 
modification  of  the  Lancashire  boiler,  the  long  tubes  described  above 
are  broken  up  by  cross  tubes.  The  object  of  this  arrangement  is  to 
increase  the  heating  surface,  where  the  hot  gases  and  the  water  are 
brought  close  to  each  other.  In  a  later  form  of  Lancashire  boiler, 
the  main  flues  have  a  number  of  small  tubes  arranged  vertically, 
breaking  up  the  space.  In  the  Cornish  boiler,  also  cylindrical  in 
section,  there  is  usually  one  large  cylindrical  flue  in  the  centre  of  the 
containing  vessel,  through  which  the  hot  gases  pass,  and  around 
which  the  water  circulates,  as  in  the  Lancashire  boiler.  In  both 
forms  of  boiler,  external  flues  are  formed  between  the  outside  of  the 
boiler  and  the  brickwork  in  which  it  is  set,  the  hot  gases  passing 
through  them  on  the  way  to  the  chimney.  The  Cornish  boiler  has 
gradually  given  way  to  the  Lancashire  boiler,  principally  because  of 
the  greater  heating  surface  the  Lancashire  boiler  is  able  to  expose, 
and  also  from  the  fact  that  the  Lancashire  boiler  more  readily  lends 
itself  to  the  storage  of  heat  by  reason  of  the  larger  quantity  of 
water  it  holds.  In  the  marine  boiler,  which  is  another  type  of  fire- 
tube  boiler,  designed  originally,  as  its  name  implies,  for  marine  work, 
there  are  a  number  of  small  tubes  through  which  the  hot  gases  pass, 
the  water  occupying  the  space  between  the  tubes.  The  tubes  form 
more  or  less  of  a  nest.  Messrs.  Davey  &  Paxman  make  a  form  of 
this  boiler,  shown  in  Fig.  53,  for  use  on  land.  It  is  used  in 
electricity  generating  stations.  In  all  types  of  steam  boilers  there  is 
some  form  of  furnace  which  may  either  be  completely  outside  of  the 
vessel  forming  the  boiler,  or  which  may  form  part  of  it.  The  furnace 
is  simply  a  long  grate  consisting  of  fire  bars,  upon  which  the  coal  or 
other  fuel  that  is  to  be  consumed  rests,  with  an  ashpit  below,  into 
which  the  ashes  fall,  and  having  at  its  inner  end,  the  end  removed 
from  the  entrance  to  the  furnace,  a  bridge  usually  formed  of  firebricks, 
over  which  the  hot  gases  pass,  the  bridge  itself  being  raised  to  a  white 
heat,  and  performing  an  important  part  in  the  work  of  combustion. 
At  the  front  or  entrance  to  the  furnace  are  doors,  usually  arranged 
in  pairs,  sliding  together  and  meeting  in  the  centre.  In  all  types  of 
boilers,  one  of  the  most  important  things  is  to  make  the  hot  gases 
deliver  up  as  much  of  their  heat  as  possible  to  the  water  from  which 
steam  is  being  generated,  and  in  order  to  accomplish  this,  the  gases 
are  caused  to  traverse  as  long  a  path  as  possible  on  their  way  to  the 
chimney,  passing  over  different  parts  of  the  heating  surface  in 
succession.  In  the  Lancashire  and  Cornish  boilers  this  is  accom- 
plished by  making  the  surface  over  which  the  gases  pass  as  large  as 
possible.  In  the  marine  type  the  object  is  accomplished  by  breaking 
up  the  flue  into  a  number  of  small  surfaces,  as  explained.  There  is 


ELECTRICITY  IN   MINING 


THE   GENERATION   OF  ELECTRICITY  91 

an  important  point  in  connection  with  the  matter  of  the  passage  of 
the  hot  gases  through  the  flues,  and  that  is,  the  hot  gases  obey  the 
laws  to  which  all  fluids  are  subject  in  passing  through  tubes,  etc., 
and  create  friction  where  they  rub  on  the  surface  of  the  tubes  through 
which  they  pass.  The  friction  created  varies  directly  as  the  extent 
of  the  surface  over  which  the  gases  have  to  pass,  and  as  the  square 
of  the  velocity  at  which  they  are  travelling,  hence  the  large  flue  in 
the  Cornish  boiler  was  thought  to  have  an  advantage  over  the 
smaller  flues  in  the  Lancashire  boilers,  because  the  flow  of  the  gases 
was  necessarily  throttled  less  in  the  larger  tube,  and  therefore,  as  in 
all  similar  cases,  the  velocity  in  passing  through  the  flues  would  be 
less.  The  question,  as  in  many  similar  cases,  is  a  very  difficult  one 
to  decide,  because  it  is  so  difficult  to  obtain  exact  figures,  but 
practice  appears  to  have  decided  the  question  in  favour  of  the 
Lancashire  boiler.  In  both  Lancashire  and  Cornish  boilers  the  gases 
are  frequently  given  an  additional  run,  through  flues  formed  between 
brickwork  built  round  the  boiler,  and  its  outside  shelf.  In  the  water- 
tube  boiler  the  same  effect  is  attained  as  in  the  Lancashire  boiler,  by 
interposing  a  series  of  baffles  in  the  path  of  the  gases  on  their  way 
to  the  chimney.  The  gases  passing  from  the  back  of  the  furnace  are 
made  to  run  over  the  surfaces  of  a  portion  of  the  tubes,  and  are  then 
deflected  by  a  baffle  over  the  surfaces  of  another  set  of  tubes,  and  so 
on,  so  that  the  whole  of  the  tubes  contained  in  the  boiler  are  subject 
to  the  action  of  the  gases. 

Forms  of  Water-tube  Boilers 

The  Babcock  &  Wilcox.— In  the  Babcock  and  Wilcox  boiler, 
shown  in-  Plate  IA,  there  are  rows  of  tubes  inclined  at  an  angle 
of  about  30°  from  the  horizontal  running  from  front  to  back  of  the 
boiler  space.  The  tubes  slope  downwards  from  front  to  back,  and  each 
end  of  each  tube  is  expanded  into  a  header.  The  headers  are  the 
junction  pieces  between  the  tubes,  and  they  connect  to  the  end  tubes 
at  front  and  back  leading  to  the  steam  drum.  At  the  front  a  number 
of  short  tubes  lead  from  the  top  of  the  header  to  the  under-side  of  the 
steam  drum ;  at  the  back  a  number  of  longer  tubes  connect  the  back 
headers  with  the  back  side  of  the  steam  drum.  The  tubes  themselves 
are  in  stacks,  so  many  deep,  and  so  many  wide  horizontally,  according 
to  the  size  of  the  boiler,  and  the  quantity  of  water  it  is  to  convert  into 
steam.  The  headers  at  front  and  back  are  slightly  inclined  with  a 
vertical.  They  are  at  right  angles  to  the  boiler  tubes.  Above  the 
tubes  themselves  is  the  steam  drum.  In  some  cases  there  are  two 
or  more  steam  drums.  The  office  of  the  steam  drum  is  to  act  as  a 
reservoir,  both  of  water  and  steam.  The  water  is  kept  circulating 
continually  through  the  tubes,  through  the  headers  into  the  steam 


92  ELECTRICITY  IN   MINING 

drum,  back  to  the  headers  through  the  tubes  again,  and  so  on.  At 
each  passage  through  the  drum  a  certain  quantity  of  steam  is  delivered 
up,  the  steam  rising  above  the  water  in  the  drum  in  the  usual  way, 
and  being  carried  off  by  the  steam  pipe.  The  boiler  tubes  are  arranged 
in  the  manner  described,  expanded  into  the  headers,  which  are  practical 
boxes  with  divisions,  in  order  that  in  case  of  any  boiler  tube  develop- 
ing a  leak,  or  not  doing  its  work  properly  in  any  other  way,  it  may  be 
disconnected.  The  space  for  the  hot  gases,  which  play  all  round  the 
boiler  tubes,  is  provided  by  brickwork,  which  is  built  into  a  steel 
skeleton  framework  from  the  floor  to  the  steam  drum,  and  enclosing 
the  rectangular  sectioned  space  inside,  containing  the  boiler  furnace 
and  the  ash  pit.  The  outside  is  formed  of  white  glazed  bricks,  the 
object  being  to  reduce  the  loss  by  radiation.  The  front  portion  has 
built  into  it  the  ashpit  door  at  the  bottom,  the  furnace  door  just 
above  it,  and  a  door  above  that,  giving  access  to  the  front  headers  of 
the  boiler  tubes.  The  furnace  of  the  boiler,  consisting  of  the  usual 
fire  bars,  occupies  the  usual  space  just  inside  the  furnace  door.  There 
is  the  usual  bridge  of  firebrick,  just  behind  the  fire  bars,  and  beyond 
that,  at  different  points  within  the  space  occupied  by  the  water  tubes, 
are  firebrick  baffles,  arranged  to  direct  the  hot  gases  generated  in  the 
furnace  successively  over  each  section  of  the  boiler  tubes.  In  the 
later  forms  of  Babcock  &  Wilcox  boilers,  a  superheater,  consisting  of 
a  number  of  small  tubes  through  which  the  steam  passes  on  its  way 
from  the  boiler  to  the  engine,  is  fixed  in  the  triangular  space  left 
vacant  between  the  upper  side  of  the  boiler  tubes  and  the  tubes  rising 
from  the  headers  at  the  back,  as  shown  in  Plate  IA. 

The  Stirling  Water- tube  Boiler. — The  Stirling  water- tube  boiler 
has  been  made  in  two  forms,  with  four  and  five  drums  respectively, 
as  shown  in  Fig.  54.  In  all  forms  of  this  boiler  there  are  three 
drums  at  the  top.  In  some  forms  there  are  two  drums  at  the 
bottom,  and  in  others  only  one.  In  all  forms  the  water  tubes  them- 
selves are  nearly  vertical,  those  near  the  front  of  the  boiler  being 
inclined  about  30°  from  the  vertical,  those  in  the  next  batch  slightly 
less,  and  those  in  the  rear  batch  very  little  out  of  vertical  at  all. 
There  are  always  three  lots  of  tubes.  The  front  lot  of  tubes  connect 
the  front  drum  at  the  top  with  the  front  drum  below  where  there  are 
two,  or  with  the  common  drum  where  there  is  only  one.  The  next 
lot  of  vertical  tubes  connects  the  middle  drum  at  the  top  with  one  of 
the  drums  below,  and  the  rear  lot  of  vertical  tubes  connects  the  rear 
drum  at  the  top  with  the  rear  drum  below  where  there  are  two,  and 
with  the  common  drum  where  there  is  only  one.  The  drums,  both  at 
the  top  and  bottom,  are  connected  together  by  transverse  tubes.  The 
middle  drum  at  the  top  is  the  one  from  which  steam  passes  to  the 
steam  pipe.  The  rear  drum  at  the  top  is  the  one  to  which  the  feed 
water  is  applied,  from  which  it  passes  to  the  rear  drum  below  where 


THE  GENERATION   OF   ELECTRICITY 


94  ELECTRICITY  IN   MINING 

there  are  two.  The  object  of  the  lower  drums,  in  this  form  of  water- 
tube  boiler,  is  to  get  rid  of  any  foreign  matter,  dirt,  etc.,  that  may 
come  in  with  the  feed  water,  it  being  deposited  at  the  bottoms  of 
those  drums,  from  which  it  can  be  carried  off.  The  water  from  which 
steam  is  being  generated  circulates  continuously  through  the  different 
lots  of  tubes,  from  front  to  rear,  and  vice  versa,  steam  being  delivered 
to  the  two  upper  front  drums  in  the  process.  The  arrangement  of 
the  furnace  and  the  space  for  the  hot  gases  is  very  similar  to  that  in 
Babcock  &  Wilcox,  the  whole  being  enclosed  by  firebricks  with  white 
glazed  bricks  on  the  outside,  the  drums  being  built  into  the  top.  As 
in  the  Babcock  boiler,  there  are  baffles  placed  at  different  parts  of 
the  hot  gas  space  to  direct  the  hot  gases  over  the  whole  of  the  surface 
of  the  boiler  tubes,  thus  the  hot  gases  pass  up  over  the  front  section 
of  the  tubes,  thence  across  to  the  top  of  the  next  section,  down  that 
section,  across  to  the  third  section,  up  that  section,  across  to  the  top 
of  the  fourth  section  where  there  are  four  sections  of  tubes,  down 
that  section,  and  away  to  the  chimney.  The  front  of  the  boiler  is 
occupied  as  usual  by  the  ashpit  door,  the  furnace  door,  and  a  door 
by  which  access  is  obtained  to  the  gas  space.  It  is  claimed  on 
behalf  of  the  Stirling  boiler  that  the  steam  in  the  front  section  of 
pipes — the  water  has  become  steam  when  it  reaches  this  section — is 
practically  superheated  while  passing  up  through  these  tubes,  in  the 
same  way  as  in  passing  through  a  separate  set  of  superheated  pipes. 

The  Hornsby  Water-tube  Boiler. — In  this  boiler  the  tubes 
are  fixed  something  on  the  lines  of  those  in  the  Stirling  boiler, 
but  with  an  arrangement  different  in  many  respects.  In  the  Stirling 
boiler,  as  will  be  seen  from  the  drawings,  the  tubes  are  curved 
where  they  approach  the  drums,  both  above  and  below.  In  the 
Hornsby  boiler  the  tubes  are  quite  straight,  and  they  are  expanded 
into  headers  at  top  and  bottom,  consisting  of  cylinders  arranged 
to  take  the  tubes.  The  tubes  are  arranged  in  batches,  a  certain 
number  of  tubes  with  the  header  cylinder  at  top  and  bottom 
forming  an  element,  and  the  number  of  tubes  in  each  element 
and  the  number  of  elements  being  varied  according  to  the  work 
the  boiler  is  intended  to  perform ;  that  is  to  say,  the  capacity  of  an 
individual  boiler  may  be  increased  by  adding  more  banks  of  tubes  or 
by  making  the  individual  banks  larger.  The  upper  header  cylinders 
are  longer  than  the  lower  cylinders;  with  the  exception  of  those 
belonging  to  the  bank  of  tubes  at  the  back  of  the  boiler.  These 
header  cylinders,  to  a  very  large  extent,  perform  the  office  of  the 
drums  in  the  Stirling  boiler,  the  lower  drums  acting  as  mud  or  dirt 
receptacles,  and  the  upper  assisting  to  equalize  the  steam  pressure. 
The  banks  of  tubes  are  all  inclined  about  30°  from  the  vertical, 
except  the  one  bank  at  the  back  of  the  boiler,  which  is  quite 
vertical.  There  is  a  steam  drum  fixed  between  the  upper  header 


THE   GENERATION   OF  ELECTRICITY  95 

cylinder  of  the  rear  bank  of  tubes,  and  the  upper  header  cylinder 
of  the  rear  one  of  the  inclined  banks;  and  a  steam  drum  is  con- 
nected with  the  upper  header  cylinder  of  the  rear  bank,  and  those  of 
the  front  banks  by  pipes  in  a  similar  manner  to  the  Stirling.  As 
with  the  other  water-tube  boilers,  the  whole  of  the  tubes,  furnace,  etc., 
are  enclosed  inside  the  firebrick  space  with  white  enamelled  bricks 
on  the  outside,  but,  in  addition,  both  the  upper  cylinder  headers  and 
the  steam  drum  are  inside  the  iron  or  steel  framework.  The  furnace, 
as  usual,  is  placed  in  the  front  with  the  usual  firebrick  bridge,  and 
the  gases  are  conveyed  by  means  of  baffles  up  the  front  of  the  first 
bank  of  inclined  tubes,  across  the  upper  part  of  the  first  bank  to  the 
second  bank,  down  the  front  of  the  second  bank,  across  the  lower  part 
of  the  second  bank  to  the  lower  part  of  the  third  bank,  up  the  front 
of  the  third  bank  by  a  somewhat  tortuous  course,  first  from  front  to 
rear,  then  from  rear  to  front,  then  from  front  to  rear  again,  and  thence 
to  the  flue. 

Thornycroft  Water-tube  Boiler.— Messrs.  Thornycroft  have 
developed  forms  of  water-tube  boilers  on  lines  of  their  own,  especially 
intended  for  use  on  board  ship;  but  they  have  also  been  used  on 
shore,  and  there  appears  to  be  no  reason  why  they  should  not  be. 
The  principal  feature  of  the  Thornycroft  boiler  is  the  division  of  the 
water  tubes  into  a  very  large  number  of  very  small  tubes,  in  some 
cases  as  small  as  1  inch  in  diameter,  and  in  the  majority  of  cases 
not  greater  than  3J  inches.  In  one  form  of  Thornycroft  boiler, 
there  are  two  drums  at  the  bottom  and  one  at  the  top,  and  the 
small  tubes  mentioned  connecting  the  bottom  tubes  and  the  upper 
tube  inside  an  outer  casing,  which  is  of  iron  or  steel  in  marine 
boilers,  but  which  may  be  of  firebrick  for  shore  boilers,  and  the 
upper  and  lower  drums  are  connected  outside  of  the  casing  by  very 
much  larger  tubes.  The  upper  drum  contains  steam  and  water,  and 
the  lower  drums  water  only.  The  water  passes  directly,  by  gravity, 
from  the  upper  drum  to  the  lower  drums,  meeting  with  very  little 
resistance  on  its  way.  It  then  commences  to  move  up  through  the 
nest  of  small  tubes  to  the  upper  drum,  bubbles  of  steam  being  formed 
as  the  water  ascends,  the  water  and  steam  finding  their  way  into  the 
upper  drum,  the  steam  being  delivered  to  the  steam  space,  and  the 
water  reinforcing  that  already  in  the  drum.  The  small  tubes  are 
bent  into  the  form  of  an  arch  over  the  furnace,  and  there  subjected  to 
the  hot  gases  arising  from  the  furnace  which  pass  over  and  between 
the  tubes,  and  finally  escape  to  the  chimney.  A  sort  of  wall  is 
formed  on  the  outside  of  the  tubes,  inside  the  casing,  by  two  rows  of 
tubes  placed  very  close  together.  There  are  several  variations  of  this 
form  of  boiler,  but  the  above  are  the  main  lines.  The  idea,  it  will  be 
seen,  is  to  divide  the  water  up  as  much  as  possible,  and  to  provide 
that  a  small  quantity  is  always  close  to  some  portion  of  the  flue 


96  ELECTRICITY  IN   MINING 

gases,  and  only  separated  from  them  by  the  thickness  of  the  pipe. 
In  another  form  of  Messrs.  Thorny  croft's  boilers,  there  is  one 
steam-and- water  drum  at  the  top,  and  at  the  back  there  is  a  vertical 
water  tank  from  which  tubes  are  brought  to  the  front  of  the  boiler, 
where  they  are  connected  in  pairs  by  junction  pieces,  the  junction 
pieces  and  the  water  tank  taking  the  place  of  the  headers  in 
the  Babcock  boiler.  The  tubes  in  one  form  of  this  boiler  are 
staggered,  and  the  furnaces  are  right  underneath  the  lowest  of  the 
tubes,  so  that  the  hot  gases  rise  between  the  tubes,  the  particular 
arrangement  of  which  causes  them  to  act  in  the  same  manner  as  the 
baffles  described  in  connection  with  other  boilers,  and  to  direct  the 
hot  gases  to  all  parts  of  the  tubes.  The  water  circulates  through 
the  tubes  and  their  connecting-pieces  and  the  water  tank,  steam  as  it 
is  formed  gradually  rising  in  the  water  tank  and  passing  by  the 
upper  tubes  to  the  steam-drum.  In  another  form  of  Messrs.  Thorny- 
croft's  boiler  the  arrangement  of  the  tubes  is  slightly  different,  and 
there  is  no  water  tank.  The  boiler  is  made  in  sections,  each  section 
consisting  of  a  certain  number  of  tubes  placed  vertically  one  above 
the  other,  expanded  into  vertical  headers  at  the  front  end  of  the 
boiler,  and  joined  to  corresponding  tubes  of  the  next  section  of  the 
boiler  at  the  rear  end.  The  alternate  sections  of  the  boiler  are 
arranged  at  different  inclinations,  thus  the  tubes  of  the  section  on  the 
extreme  left  may  be  inclined  to  the  horizontal  at  about  15°,  the  front 
being  below  the  back,  while  the  next  sections  are  inclined  upwards 
about  the  same  number  of  degrees.  The  headers  in  the  front  are 
connected  to  the  steam  drum,  and  there  is  a  continual  circulation  of 
water  through  the  pairs  of  sections  of  tubes,  the  steam  as  it  is  formed 
being  disengaged  in  the  front  headers  and  escaping  into  the  steam - 
drum.  The  furnace  lies  right  under  the  tubes,  and  the  gases  escape 
between  the  different  sections. 

The  Niclausse  is  a  French  boiler,  which  was  taken  up  and  manu- 
factured by  Messrs.  Willans  &  Kobinson.  It  is  rather  like  the 
Babcock  in  many  respects,  and  has  been  largely  used  in  the  French 
navy.  The  principal  feature  of  it  is  the  arrangement  of  the  tubes. 
These  are  inclined  slightly  to  the  horizontal,  are  expanded  into 
headers  in  the  front,  but  are  only  held  at  the  back,  not  expanded 
into  headers,  the  arrangement  consisting  of  one  tube  inside  another, 
the  water  going  down  the  inside  tube  and  returning  by  the  annular 
space  between  the  tubes.  So  far  as  the  writer  is  aware,  the  Niclausse 
boiler  has  not  been  much  used  in  this  country. 

The  Climax  is  an  American  boiler,  and'  is  in  use  in  a  considerable 
number  of  electrical  generating  stations  in  this  country,  and  in  a  very 
large  number  of  works  of  all  kinds  in  America.  In  this  boiler  a 
distinctly  new  line  has  been  struck.  It  is  essentially  a  water-tube 
boiler,  but  the  tubes  are  of  very  peculiar  arrangement  and  construction. 


THE   GENERATION   OF   ELECTRICITY 


97 


Looking  at  the   boiler  with   the  outer  casing  removed,  one  would 
imagine   that  one  was  looking  almost   at  a  coil   of  rope.     There 


RETURN  BEND 
VERTICAL  CHECH 
REVERSED 
FOR  RECULATOB 


CLCBE  YAUYE. 


WATER  LINE 


FIG.  55.— Half  Vertical  Transverse  Section,  and  Half  External  View  of  "  Climax" 
Boiler.    A  is  the  Central  Tube  to  which  all  the  Spiral  Tubes  are  connected. 

is  a  central  vertical  cylinder,  which  passes  from  the  firebox  to  the 
steam  pipe  at  the  top.     This  corresponds  to  the  steam  drum  in  other 

H 


98  ELECTRICITY   IN   MINING 

types.  The  vertical  cylinder  has  holes  drilled  in  it,  in  parallel  rows, 
throughout  its  length,  and  to  these  holes  are  connected  the  tubes  of 
the  boiler,  which  are  comparatively  small  in  section,  and  are  made  in 
a  spiral  form.  Each  tube  passes  from  a  hole  in  the  vertical  cylinder 
at  a  certain  height,  and  after  performing  a  certain  convolution  with 
the  other  tubes,  enters  the  vertical  tube  again  lower  down.  The  whole 
of  the  boiler  forms  a  vertical  cylinder  when  the  casing  is  on,  and 
the  furnace  and  ashpit  occupy  the  whole  of  the  lower  portion  of  the 
cylinder,  the  hot  gases  passing  up  and  circulating  between  interstices 
left  by  the  convolutions  of  the  tubes,  and  therefore  playing  over  all 
parts  of  their  surfaces,  and  passing  thence  to  the  funnel.  There  are 
four  firebrick  doors  at  points  90°  apart  in  the  lower  part  of  the 
cylinder,  and  the  furnace  is  enclosed  inside  a  wrought-iron  casing 
lined  with  firebrick.  The  boiler  occupies  less  floor  space  for  a  given 
evaporation  per  hour  than  most  boilers.  For  purposes  of  cleaning, 
the  outer  casing  is  pierced  with  doors  about  45°  apart  at  different 
levels,  according  to  the  sizes  of  the  boiler,  and  it  is  usual  to  fix 
galleries  at  these  levels,  built  of  open  ironwork,  so  that  any  part  of 
the  boiler  -may  be  got  at  for  cleaning  purposes.  It  is  shown  in 
Fig.  55  and  Plate  IB. 

What  Combustion  is 

Before  considering  the  sources  of  economy  in  connection  with  steam 
raising,  it  may  be  as  well  to  study  combustion  itself.  Coal,  as  is  well 
known,  contains  carbon,  hydrogen,  and  other  elements,  and  by  combus- 
tion we  mean  the  combination  of  the  carbon  and  the  hydrogen  with  the 
oxygen  of  the  atmosphere.  The  combination  of  carbon  with  oxygen 
liberates  heat,  10,000  heat  units  approximately  per  pound  of  carbon, 
when  one  atom  of  carbon  combines  with  one  atom  of  oxygen  to  form 
carbonic  oxide,  and  a  further  4000  heat  units  approximately  when 
the  carbonic  oxide  combines  with  a  second  atom  of  oxygen  to  form 
carbonic  acid,  or  the  whole  14,000  units  will  be  liberated  when  the 
carbon  unites  with  the  two  atoms  of  oxygen  at  once,  and  perfect 
combustion  of  the  carbon  is  obtained  when  the  whole  of  the  carbon 
is  oxidized  to  carbonic  acid ;  but  in  the  ordinary  working  of  a  boiler 
furnace  this  is  very  rarely  accomplished.  Some  carbonic  acid  is 
formed,  some  carbonic  oxide,  and  some  carbon  is  carried  away  in 
a  very  finely  divided  -state,  and  appears  later  as  smoke,  having  con- 
tributed nothing  to  the  general  heating  effect.  The  combination  of  1  Ib. 
of  hydrogen  with  oxygen,  in  the  proportion  of  two  atoms  of  hydrogen 
to  one  atom  of  oxygen  liberates  approximately  62,000  heat  units,  and 
therefore  fuels  which  are  rich  in  hydrogen  have  a  higher  calorific 
value ;  that  is,  they  liberate  more  heat  in  the  process  of  combustion 
than  fuels  which  are  not  so  rich.  This  is  one  reason  why  petroleum 


THE    GENERATION    OF   ELECTRICITY 


99 


has  a  higher  calorific  value,  for  equal  weights,  than  coal.  But  the 
above  does  not  represent  the  whole  case.  Coal  very  rarely  consists 
simply  of  carbon  and  hydrogen.  Oxygen  is  also  nearly  always  present, 
and  very  frequently  other  substances,  such  as  sulphur,  and  what  are 
generically  known  as  dirt.  The  oxygen  contained  in  the  coal  practi- 
cally represents  a  certain  loss  of  calorific  value,  inasmuch  as  it  will 
absorb  a  certain  quantity  of  hydrogen,  the  combination  forming  water 
in  the  usual  way,  every  atom  of  oxygen  absorbing  two  atoms  of 
hydrogen,  the  number  of  atoms  of  hydrogen  thus  absorbed  in  satisfy- 
ing the  demands  of  the  oxygen  contained  in  the  coal  being  lost,  so  far 
as  heating  value  is  concerned.  In  addition,  when  coal  contains 
sulphur,  that  also  absorbs,  or  may  absorb,  a  certain  quantity  of 
hydrogen,  forming  sulphuretted  hydrogen,  the  effect  of  which  upon 
the  final  calorific  value  is  doubtful.  Where  dirt  is  carried  in  the 
coal,  it  absorbs  a  certain  quantity  of  the  heat  liberated,  in  proportion 
to  its  weight  and  its  specific  heat.  The  dirt,  using  the  term  to  mean 
substances  that  cannot  be  usefully  burned,  that  do  not  combine 
usefully  with  oxygen,  unless  they  are  carried  harmlessly  into  the 
ashpit,  and  until  they  are  carried  there,  or  are  carried  up  the  chimney, 
must  necessarily  be  made  to  assume  the  temperature  of  the  bodies 
surrounding  them,  and  in  so  doing  must  absorb  heat  from  the  hot 
gases  that  are  formed,  and  so  take  from  the  useful  heating  effect  of 
those  gases.  When  the  coal  is  burned,  the  gases  which  are  formed  are 
raised  to  a  very  high  temperature,  and  it  is  these  hot  gases  which  are 
made  to  pass  through  the  flues'and  pipes  and  over  the  tubes  described 
above,  and  which  deliver  a  portion  of  the  heat  which  has  been 
delivered  to  them,  to  the  metal  surfaces  over  which  they  rub,  the 
heat  being  conveyed  from  the  metal  plates  to  the  water  which  is 
moving  over  the  other  sides  of  the  metal  plates  or  tubes. 

The  British  Thermal  Unit 

Perhaps  before  going  any  further  we  had  better  understand  what 
is  meant  by  the  heat  unit,  or,  as  it  is  termed  in  scientific  phraseology, 
the  British  Thermal  Unit.  It  is  the  standard  which  is  adopted  for 
all  measurements  in  which  the  transference  of  heat  from  one  body  to 
another  takes  place.  The  British  Thermal  Unit  is  that  quantity  of 
heat  which  will  raise  the  temperature  of  1  Ib.  of  pure  water,  at  its 
maximum  density  39'2°  Fahr.,  1°  Fahr.  Strictly  speaking,  the 
quantity  of  heat  required  to  raise  the  temperature  of  1  Ib.  of  water 
1°  Fahr.  increases  slightly  as  the  temperature  of  the  water  in- 
creases; that  is  to  say,  the  quantity  of  heat  required  to  raise  the 
temperature  of  1  Ib.  of  water  at,  say,  from  80°  Fahr.  to  81°  Fahr.  is  a 
little  more  than  that  required  to  raise  its  temperature  from  39°  Fahr. 
to  40°  Fahr.  For  practical  purposes,  however,  the  quantity  of  heat 


ioo  ELECTRICITY   IN   MINING 

required  to  increase  the  temperature  of  1  Ib.  of  water  through  each 
successive  degree  Fahrenheit  between  freezing-point  and  boiling-point 
of  water  is  taken  to  be  the  same,  and  this  quantity  is  taken  as  the 
standard.  The  specific  heat  of  any  substance  is  the  ratio  between  the 
quantity  of  heat  required  to  raise  1  Ib.  of  the  substance  1°  Fahr.,  and 
that  required  to  raise  1  Ib.  of  water  1°  Fahr.  The  specific  heat  of 
water  is  taken  as  unity,  and  as  the  specific  heats  of  the  great  majority 
of  other  substances  are  less  than  that  of  water,  they  are  usually 
written  as  decimals.  The  specific  heat  of  air,  for  instance,  is  0*238 
at  constant  volume,  and  0*169  at  constant  pressure.  This  matter  will 
be  dealt  with  later.  The  matter  of  the  heat  unit,  it  will  be  found, 
will  follow  us  through  almost  every  problem  that  we  shall  discuss  in 
the  course  of  this  book.  The  British  Thermal  Unit  has  its  direct 
mechanical  equivalent ;  that  is  to  say,  the  energy  in  one  heat  unit  is 
equivalent  to  the  energy  of  778  foot-lbs.  We  shall  find  later  on, 
when  discussing  the  problems  involved  in  electrical  distribution,  that 
the  heat  unit  will  come  up  again.  The  electric  current  delivers  heat 
to  the  conductors  through  which  it  passes  in  definite  quantities,  each 
electrical  unit  delivering  a  certain  fraction  of  a  heat  unit,  and  so  on. 

But  in  following  the  course  of  the  increase  of  temperature  of  water 
and  its  conversion  into  steam,  we  have  to  take  notice  of  another 
property,  known  as  latent  heat.  If  we  take  1  Ib.  of  water  and  apply 
heat  to  it,  raising  its  temperature  degree  by  degree,  we  find  that 
approximately  one  heat  unit  is  absorbed  for  every  degree  of  increase 
in  temperature,  but  when  steam  commences  to  be  formed  from  the 
water,  the  temperature  of  the  steam  and  of  the  water  remain  exactly 
the  same,  212°  Fahr.  at  ordinary  barometric  pressures,  until  the 
whole  of  the  pound  of  water  has  been  converted  into  steam,  and 
the  conversion  of  the  pound  of  water  into  a  pound  of  steam  at 
atmospheric  pressure  absorbs  966  heat  units,  and  this  is  the  latent 
heat  of  steam  at  that  pressure.  At  the  other  end  of  the  scale,  when 
water  is  frozen,  there  is  a  very  similar  operation,  but  in  the  reverse 
order.  If  we  wish  to  freeze  a  pound  of  water,  we  have  to  abstract 
from  the  water  the  latent  heat  of  the  liquid,  which  is  142  heat 
units,  and  while  the  process  of  abstracting  the  heat  units  is  going 
on,  the  temperature  of  the  water  and  the  temperature  of  the  ice 
which  is  being  formed  from  it  remain  the  same,  -viz.  32°  Fahr.  at 
ordinary  atmospheric  pressure.  When  all  the  water  has  been  con- 
verted into  steam,  the  temperature  of  the  steam  rises  and  also  its 
pressure,  as  will  be  explained,  unless  the  steam  is  able  to  escape  and 
expend  the  energy  that  has  been  delivered  to  it.  Similarly,  after  the 
whole  of  the  water  has  been  frozen,  if  the  process  of  abstraction  of 
the  heat  is  continued,  the  temperature  of  the  ice  itself  is  lowered. 
The  temperature  of  the  natural  ice  which  is  formed  in  rivers,  glaciers, 
etc.,  is  usually  a  great  deal  below  freezing-point,  and  the  ice  which  is 


THE   GENERATION   OF  ELECTPtlClTY  101 

formed  artificially  by  refrigerating  machinery  is  also  usually  made  a 
good  many  degrees  below  freezing,  so  that  it  may  remain  firm  and 
solid,  and  not  become  sloppy.  It  was  mentioned  above  that  the 
combination  of  1  Ib.  of  carbon  with  oxygen  in  the  proportion  of  one 
atom  of  carbon  to  two  atoms  of  oxygen  liberated  approximately 
14,000  heat  units.  These  are  the  British  Thermal  Units  just 
described.  It  will  be  seen,  therefore,  that  there  is  a  direct  connection 
between  the  combustion  of  coal  and  the  raising  of  steam.  By  the 
combustion  of  coal,  a  certain  number  of  heat  units  are  liberated, 
according  to  the  composition  of  the  coal.  These  heat  units  are 
delivered  to  the  gases  which  are  formed  in  the  process  of  combustion, 
and  a  portion  of  the  heat  delivered  to  them  is  passed  on  to  the  water 
in  the  tubes,  or,  surrounding  the  tubes,  in  the  boilers.  The  great 
object  the  boilermaker  has  in  view  is,  the  transmission  of  the  largest 
portion  of  the  heat  present  in  the  gases  produced  by  combustion,  to 
the  water,  and  to  the  steam  in  the  boiler.  As  with  all  machinery, 
under  the  very  best  conditions,  the  whole  of  the  heat  cannot  be 
transmitted  to  the  water ;  and  in  practice,  what  is  known  as  the 
efficiency  of  the  boiler,  that  is  to  say,  the  proportion  of  the  heat 
liberated  that  is  transmitted  to  the,  water  is  comparatively  small,  and 
for  various  reasons.  The  calorific  value  of  coal  varies  from  5000 
units  per  pound  of  coal  for  brown  coal  lignite,  and  similar  coals  up  to 
14,000  heat  units  per  pound  for  best  anthracite.  By  calorific  value 
is  meant  the  number  of  heat  units  obtained  by  the  combustion  of 
1  Ib.  of  coal  in  oxygen.  The  standard  value  for  calculations  is  taken 
at  10,000  heat  units  per  pound  of  coal.  In  describing  the  process  of 
combustion  above,  the  quantities  of  heat  liberated  by  the  combination 
of  a  pound  of  carbon,  and  a  pound  of  hydrogen  with  oxygen  re- 
spectively, was  given ;  but  those  figures  were  on  the  assumption  that 
pure  oxygen  was  available  for  the  process  of  combustion.  In  practice 
the  oxygen  of  the  air,  which  is  diluted  with  approximately  four 
times  its  weight  of  nitrogen,  a  perfectly  inert  gas,  is  used  in  combus- 
tion, and  the  quantity  of  nitrogen  which  is  admitted  to  the  furnace 
with  the  oxygen  of  the  atmosphere,  has  also  to  be  heated  to  the 
temperature  of  the  other  products  of  combustion,  carbonic  acid,  etc. 
And  here  it  will  be  seen  where  the  value  of  the  knowledge  of  specific 
heat  and  the  use  of  the  British  Thermal  Unit  comes  in.  Nitrogen 
has  a  specific  heat  of  0*2754,  and  therefore  every  pound  of  air  which 
is  delivered  to  the  boiler  furnace  brings  with  it  approximately  four- 
fifths  of  a  pound  of  nitrogen  gas,  demanding  four-fifths  multiplied  by 
0'2754  for  every  degree  that  its  temperature  is  raised.  As  the 
furnace  temperature  is  usually  in  the  region  of  3000°  Fahr.,  every 
pound  of  air  admitted  to  the  furnace  brings  with  it,  in  the  nitrogen 
forming  part  of  it,  a  demand  of  638  heat  units  to  raise  the  temperature 
of  the  nitrogen  to  that  of  the  other  bodies  surrounding  it,  and  this 


102  ELECTRICITY   IN   MINING 

number  of  heat  units  is  taken  from  those  which  are  liberated  by  the 
combustion  of  the  coal.  And  this  is  only  one  of  the  sources  of  loss 
of  heat  which  takes  place  in  the  boiler  furnace.  Theoretically, 
12 '2  Ibs.  of  air  are  sufficient  for  the  combustion  of  1  Ib.  of  carbon, 
but  it  is  found  necessary  to  admit  from  18  to  22  Ibs.  of  air  per  pound 
of  coal  consumed  with  ordinary  chimney  draught.  As  the  carbon 
and  hydrogen  of  the  coal  can  only  combine  with  the  definite 
quantities  of  oxygen  that  have  been  named,  the  nitrogen  remaining 
over  from  the  air,  from  which  the  oxygen  of  combustion  is  taken, 
and  the  whole  of  the  surplus  air  supplied,  all  demand  heat  from  that 
liberated  by  the  combustion  of  the  coal,  to  raise  them  to  the  tempera- 
ture of  the  surrounding  bodies.  The  air  and  the  free  nitrogen  gas 
which  is  so  heated,  form  part  of  the  hot  gases  mentioned  above,  as 
passing  through  the  flues  and  round  the  pipes,  and  doubtless  take 
their  share  of  delivering  heat  to  the  metal  surfaces  over  which  they 
rub ;  but  the  point  is,  the  transmission  of  the  heat  from  the  coal  to 
the  metal  surfaces  could  have  been  accomplished  quite  well  by  the 
carbonic  acid  gas  formed,  if  complete  combustion  had  been  attained, 
and  if  no  diluent  had  been  present.  In  addition  to  this,  there  is 
another  peculiar  action.  However  carefully  coal  may  be  handled,  a 
certain  amount  of  dust  is  formed,  and  dust  is  created  in  the  process 
of  stoking,  and  in  the  ordinary  process  of  combustion  there  is  a 
mechanical  action  of  the  hot  gases  passing  from  the  coal  on  the 
furnace  bars  to  the  flues,  somewhat  similar  to  that  of  a  stream  of 
water  running,  say,  through  a  coal-washing  machine.  The  hot  gases 
carry  minute  particles  of  coal,  and  also  minute  particles  of  uncom- 
bined  carbon  with  them  in  their  passage,  and  these  particles  of  coal 
or  of  carbon  not  only  perform  no  useful  work  in  the  generation  of 
heat,  but  they  also  demand  that  heat  shall  be  delivered  to  them,  in 
order  that  their  temperature  may  be  raised  to  that  of  the  gases  in 
which  they  are  imprisoned,  and  this  heat  is  also  taken  from  that 
liberated  by  the  useful  combustion  of  the  coal.  And  there  is  still 
another  source  of  loss  of  heat,  or  rather  of  demand  for  heat  from  that 
liberated  by  combustion.  The  air  which  is  admitted  to  the  furnace 
is,  in  the  great  majority  of  cases,  at  a  very  much  lower  temperature 
than  the  furnace  itself.  Even  where  the  boiler  room  is  at  a  com- 
paratively high  temperature,  an  oppressive  temperature  for  the 
stokers,  the  air  passing  into  the  furnace  from  the  boiler  room  is  a 
great  many  hundred  degrees  lower  in  temperature  than  the  mass  of 
burning  coal  on  the  tire  bars,  and  the  first  thing  which  happens  when 
air  is  admitted,  as  when  the  furnace  doors  are  opened  for  the  purpose 
of  stoking,  or  in  connection  with  the  air  passing  through  the  ashpit, 
and  thence  through  the  fire  bars  to  the  burning  coal,  is,  the  air  itself 
absorbs  a  large  quantity  of  heat,  directly  in  proportion  to  its  weight 
and  to  its  specific  heat,  or,  say,  approximately  a  quarter  of  a  heat  unit 


THE   GENERATION   OF  ELECTRICITY          103 

for  every  pound  of  air  admitted ;  and  this,  again,  has  to  be  taken  from 
the  heat  liberated  in  combustion.  Hence  while,  on  the  one  hand,  there 
is  often  a  difficulty  in  obtaining  a  sufficient  quantity  of  air  through  the 
ashpit  for  anything  approaching  complete  combustion,  on  the  other 
hand  the  air  entering  by  the  fire  doors  each  time  they  are  opened  has 
a  distinct  damping  effect.  The  effect  of  suddenly  admitting  a  volume  of 
air  at,  say,  80°  Fahr.  on  to  a  mass  of  coal  at  approximately  3000°  Fahr. 
is  somewhat  similar  to  the  effect  of  throwing  water  upon  a  hot  body. 
It  lowers  the  temperature  of  the  mass  of  gases  just  rising  from  the 
coal;  it  tends  to  increase  the  quantity  of  finely  divided  carbon 
coming  away ;  and  so  on. 

Apparatus  for  Economizing  in  Coal  and  Labour 

Mechanical  Stokers. — There  is  a  good  deal  of  waste  of  coal 
with  hand-stoking  in  the  great  majority  of  cases,  because  whenever 
the  furnace  doors  are  open  a  quantity  of  cold  air  passes  into  the 
furnace.  When  the  boiler  furnace  is  fed  by  hand,  no  matter  how 
carefully  it  may  be  done,  there  is  always  a  certain  quantity  of  loose 
coal-dust  entering  with  the  lumps  of  coal,  and  a  large  portion  of  this 
loose  dust  is  carried  off  with  the  hot  gases.  It  is  nearly  always 
possible  to  see  when  a  boiler  furnace  is  being  stoked  by  the  sudden 
excess  of  black  smoke  from  the  chimney.  Hence  one  source  of 
economy  is  the  mechanical  stoker.  Stoking  is  an  art  requiring 
considerable  skill,  care,  and  experience.  It  is  doubtful  whether  even 
the  best  mechanical  stokers  can  compete  with  a  really  good,  careful 
stoker  who  knows  his  work,  and  who  is  handling  a  good  quality  of 
fuel ;  but  every  practical  mechanical  stoker  is  very  much  better  than 
the  poor  stoker,  and  is  generally  better  than  the  average  stoker, 
which  is,  after  all,  what  the  engineer  has  to  work  to.  In  addition, 
the  mechanical  stoker  enables  a  very  much  lower  quality  of  coal  to 
be  employed.  Even  the  very  best  and  most  careful  stoker  is  not 
able  to  avoid  waste  with  low  grades  of  fuel. 

Forms  of  Mechanical  Stoker. — The  mechanical  stoker  which 
is  to  successfully  take  the  place  of  hand-firing  must  perform  two 
distinct  functions.  It  must  feed  the  coal  to  the  furnace  in  quantities 
such  that  the  furnace  can  deal  with  it  and  consume  it  economically, 
and  it  must  also  perform  the  operations  known  generally  as  stoking, 
performed  by  the  stoker  with  the  shovel  and  other  tools,  the  object 
of  which  is  to  present  different  parts  of  the  fuel  to  the  hottest  part 
of  the  fire  in  succession,  and,  by  keeping  the  fuel  more  or  less  in 
motion,  to  secure  complete  combustion.  There  are  three  main  lines 
upon  which  mechanical  stokers  are  designed.  With  all  of  them  the 
coal  is  placed  in  a  hopper,  directly  over  the  entrance  to  the  furnace, 
the  hopper  having  a  valve  where  it  leads  into  the  furnace,  the  valve 


104  ELECTRICITY  IN   MINING 

being  opened  periodically,  and  a  certain  quantity  of  the  fuel  being 
ejected  from  the  hopper  on  to  the  furnace  bars.  In  one  form  of 
mechanical  stoker  the  bars  are  arranged  in  two  sets,  alternate  bars 
forming  one  set,  and  at  certain  intervals  one  set  of  bars  is  pulled 
up,  the  other  set  being  depressed,  the  rising  of  the  one  set  and  the 
depression  of  the  other  tending  to  disturb  the  whole  of  the  fuel 
upon  the  bars,  and  to  give  it  a  movement  forward.  In  another 
form  of  stoker,  the  bars  are  given  a  jerk  periodically,  with  the 
same  object.  Plates  2 A,  B,  c  show  Hodgkinson's  mechanical  stoker, 
which  is  constructed  on  these  lines.  In  the  Babcock  chain  stoker 
a  different  arrangement  rules.  The  whole  of  the  furnace  bars  are 
formed  into  an  endless  chain  of  deep  links,  what  would  be  breadth 
in  a  link  of  an  ordinary  chain  being  depth  in  the  case  of  the 
chain  stoker.  The  endless  chain  passes  over  rollers  in  the  front 
of  the  furnace  and  at  the  back,  as  well  as  at  different  parts  of  its 
length,  so  as  to  reduce  friction.  The  fuel  is  ejected  on  to  the  surface 
of  the  chain  from  the  hopper,  in  the  same  manner  as  with  the 
others  that  have  been  mentioned,  and  is  gradually  carried  forward 
by  the  motion  of  the  chain  itself.  The  chain  is  continually  moving, 
so  that  its  upper  portion,  which  forms  the  fire  grate,  is  moving 
towards  the  back  of  the  furnace,  and  its  lower  portion,  which 
passes  under  the  fire  grate,  and  which  forms  the  return  half  of 
the  chain,  is  moving  towards  the  front  of  the  fire  grate. 


Natural,  Induced,  and  Forced  Draught 

It  is  well  understood  that  it  is  necessary  to  have  a  chimney 
in  connection  with  a  boiler  furnace,  for  the  same  reason  as  it  is 
necessary  to  provide  an  upcast  shaft  to  a  coal-pit.  As  explained  above, 
for  combustion  a  certain  quantity  of  air  must  be  passed  through 
the  coal  on  the  fire  bars,  to  provide  the  oxygen  gas  required  by  the 
carbon  and  hydrogen.  This  air  enters  by  the  ashpit  and  by  the  fire 
doors  when  open,  and  passes  through  the  coal  and  the  hot  gases 
which  are  formed,  after  passing  through  the  boiler  flues,  enter  the 
boiler  chimney  and  escape  at  the  top.  The  measure  of  the  difference 
in  weight  between  the  column  of  hot  gases  in  the  boiler  chimney, 
and  that  of  an  equivalent  column  of  atmospheric  air  outside  is 
the  "motive  column/'  creating  the  natural  draught  by  which  the 
supply  of  air  to  the  furnace  is  provided.  It  is  therefore  necessary 
that  the  gases  shall  be  at  a  certain  minimum  temperature  when 
they  pass  into  the  bottom  of  the  chimney.  The  case  is  exactly 
the  same  as  that  of  mine  ventilation  by  furnace.  But  in  the  majority 
of  boiler  installations  the  temperature  at  which  the  gases  escape  into 
the  chimney,  is  considerably  higher  than  is  necessary  to  provide 


PLATE  2A.— Hodgkinson's  Mechanical  Stoker.     View  from  Stokehole,  showing 
Hoppers,  and  Driving  Gear. 


PLATE  2B.— Hodgkinson's  Mechanical  Stoker.     View  from  Back  of  Boiler,  showing  Furnace  Bars. 
It  will  be  noticed  that  one  Set  of  Bars  are  level,  while  the  others  have  Alternate  Bars  displaced. 


PLATE  2c.— Longitudinal  Section  of  Lanca- 
shire Boiler  fitted  with  Hodgkinson's 
Mechanical  Stoker.  The  Hopper  and  Gear- 
ing are  seen  on  the  Left. 


PLATE  2o.— Carter's  Economizer,  showing  the 
Arrangement  of  the  Tubes,  two  in  each 
Casting,  and  the  Scraping  Apparatus. 

[To  face  p.  104. 


THE    GENERATION   OF   ELECTRICITY  105 

sufficient  draught,  hence  a  not  inconsiderable  portion  of  the  heat 
liberated  by  the  combustion  of  the  coal  is  earned  uselessly  up  the 
chimney.  In  addition  to  this,  the  similarity  of  the  problem  of  providing 
draught  for  boiler  furnaces,  and  of  providing  a  ventilating  air  current 
for  a  coal-mine,  will  have  presented  itself  to  mining  engineers ;  and 
just  as  mining  engineers  have  displaced  the  ventilating  furnace  by  a 
fan,  so  boiler  engineers  are  gradually  moving  in  the  same  direction. 
It  will  be  understood  that,  as  in  every  engineering  problem,  it  is  a 
question  of  the  balance  sheet.  If  a  chimney  is  depended  upon  for 
draught,  the  cost  of  creating  the  draught  may  be  taken  to  be  the 
interest  on  the  cost  of  the  chimney,  plus  the  amount  necessary  to 
be  spent  on  its  upkeep  and  upon  clearing  out  the  soot  periodically, 
plus  the  heat  wasted  in  the  hot  gases.  The  chimney  may  be  replaced 
by  a  fan,  acting  either  by  suction  or  by  pressure,  just  as  the  old 
coal-pit  furnace  has.  With  the  suction  fan,  what  is  called  induced 
draiight  is  employed.  The  fan  is  placed  near  the  base  of  the 
chimney,  and  the  hot  gases  are  sucked  through  it,  just  as  the 
return  air  is  sucked  through  the  fan  at  the  top  of  the  upcast 
pit.  With  this  arrangement  there  is  no  necessity  for  any  special  air 
supply  at  the  front  of  the  boiler,  except  to  ensure  that  there  is  always 
a  sufficient  quantity  available.  There  is,  however,  the  obvious 
difficulty  attendant  upon  using  a  fan  through  which  gases  at  the 
very  high  temperature  at  the  back  of  the  boiler  must  pass,  and  that 
of  the  constant  deposit  of  finely  divided  carbon  in  the  air  passages. 
The  pressure  fan  is  placed  at  the  front  of  the  boiler,  air  ducts 
are  laid  to  the  ashpits,  and  the  air  is  forced  into  the  ashpit  and 
up  through  the  coal  on  the  furnace  bars,  just  as  a  pressure  fan 
at  the  top  of  the  downcast  pit  forces  air  into  a  coal-mine.  This  is 
called  "  forced  draught."  The  fan  has  also  been  replaced  by  steam 
jets.  An  air  duct  is  placed  in  the  side  of  the  boiler  leading  right 
under  the  furnace  bars  near  the  back,  and  a  steam  jet  is  fixed  in  the 
centre  of  this  duct,  the  jet  impinging  on  to  the  furnace  bars  above. 
The  steam  from  the  jet  condenses  on  the  lower  side  of  the  furnace 
bars,  and  in  doing  so  creates  a  difference  of  pressure  between  the  air 
at  the  furnace  bars  and  the  air  in  the  boiler  house  outside,  the  result 
being  that  there  is  a  continual  passage  of  air  through  the  duct, 
which  is  bell-mouthed  on  the  inside,  and  the  furnace  bars,  the  effect 
being  the  same  as  that  created  by  either  the  pressure  or  the  suction 
fan.  In  addition  to  creating  a  draught,  the  steam  jet  exercises  a 
cooling  effect  upon  the  furnace  bars,  thereby  reducing  their  wear. 
The  steam  which  is  at  first  condensed  upon  the  bars  is  afterwards 
re-evaporated,  and  the  heat  required  for  evaporation  is  taken,  as 
usual  in  these  cases,  very  largely  from  the  bars  upon  which  the 
condensed  steam  is  resting.  Again,  it  is  a  question  of  a  balance 
sheet  between  the  method  of  creating  a  draught  by  a  jet  of  steam, 


io6  ELECTRICITY   IN   MINING 

and  by  either  of  the  fans.  On  the  one  hand,  there  is  the  cost  of 
the  steam  used,  and  the  interest  on  the  cost  of  fixing  it  to  the  boiler ; 
on  the  other  side  is  the  interest  on  the  cost  of  fixing  the  fans  and 
whatever  may  be  employed  to  drive  them,  plus  their  upkeep,  and 
plus  the  cost  of  driving  them.  With  either  form,  however,  the 
height  of  the  chimney  may  be  very  considerably  reduced.  All  that 
is  necessary  where  induced,  forced,  or  steam-set  draught  is  employed 
is,  that  the  chimney  shall  be  of  sufficient  height,  and  its  sectional 
area  sufficiently  great,  to  carry  off  any  carbon  particles,  smoke,  etc., 
that  may  pass  through  it,  at  a  height  sufficient  to  prevent  their  being 
a  nuisance.  Where  the  chimney  is  already  built,  as  in  so  many 
cases,  it  is  not  so  easy  to  show  an  economy  by  the  addition  of  fans 
or  jets,  or  other  means  of  providing  additional  draught.  The  great 
drawback  to  the  chimney  is,  in  certain  cases,  the  fact  that  its  ability 
to  create  a  draught  is  limited.  When  all  the  dampers  are  out  and 
the  chimney  is  doing  its  utmost,  the  draught  created  is  the  largest 
possible  without  the  addition  of  fans  or  steam  jets.  On  the  other 
hand,  the  conditions  required  for  the  economical  burning  of  low- 
grade  fuels  and  of  coal-dust  and  similar  substances  are  that 
considerable  draught  shall  be  obtainable. 

Methods  of  Burning  Low-grade  Fuel 

There  are  two  principal  forms  of  cheap  fuel  that  it  will  pay 
colliery  owners  to  burn,  if  they  can  do  so  without  increasing  the 
running  costs  of  the  colliery  in  other  respects,  viz.  the  coal  which  is 
unsaleable,  that  which  exists  very  often  on  the  outside  of  a  coal  seam 
or  between  two  seams,  and  the  dust  which  is  recovered  after  washing 
the  coal  for  coking,  and  other  purposes.  There  are  two  methods  of 
burning  the  bad  coal — that  which  is  largely  mixed  with  dirt.  One 
is  by  grinding  it  up  to  a  very  fine  powder  and  driving  it  into  the 
furnace  in  a  cloud  mixed  with  air,  the  other  is  by  burning  it  simply 
on  special  forms  of  grates  provided  for  the  purpose.  Messrs.  Meldrum 
of  Manchester,  who  have  worked  in  the  refuse  destructor  field,  have 
developed  a  form  of  fire  grate  which  they  claim  will  burn  practically 
any  kind  of  coal,  or,  in  fact,  any  kind  of  fuel  whatever.  The  fire 
bars  are  made  in  short  lengths  of  a  special  section,  and  fitted  very 
closely  together,  two  or  three  consecutive  bars  going  to  the  length  of 
the  grate,  the  bars  interlocking  with  each  other.  Every  method 
of  burning  low-grade  fuels,  or  fuel  dust  of  any  kind,  requires  the 
provision  of  a  considerable  draught,  very  much  in  excess  of  that 
necessary  with  lamp  fuel.  In  order  that  the  fine  dust  and  the  low- 
grade  fuel  may  be  consumed,  the  air  must  be  brought  into  contact 
with  every  portion  of  it,  and  this  can  only  be  done  by  driving  air 
through  its  mass.  On  the  other  hand,  the  draught  must  not  be 


THE   GENERATION    OF  ELECTRICITY 


107 


sufficient  to  carry  the  dust 
through  bodily.  In  Messrs. 
Meldrum's  apparatus  the 
draught  is  supplied  by  a  jet 
of  steam,  as  explained  on 
p.  105.  The  steam  is  provided 
from  the  steam  chest  of  the 
boiler  by  a  pipe  arranged  for 
the  purpose,  but  before  it  is 
allowed  to  enter  the  "blower," 
as  it  is  called,  it  is  super- 
heated by  passing  through  a 
tube  fixed  in  the  front  of  the 
furnace  over  the  dead  plate. 

The  other  plan  of  using 
dust  or  very  low-grade  fuels 
is,  it  is  placed  in  a  hopper 
in  the  same  position  as  the 
hopper  of  the  ordinary  me- 
chanical stoker,  and  a  fan  or 
other  apparatus  carries  it  into 
the  furnace.  The  coal  may 
be  ground  specially  for  the 
purpose,  as  in  the  apparatus 
known  as  the  Cyclone,  in 
which  there  is  a  special  form 
of  disintegrator,  in  which  the 
lumps  of  coal  and  dirt  are 
torn  up  by  revolving  discs, 
with  projections  upon  them, 
and  the  coal-dust  produced 
is  cleaned  from  dirt  by  centri- 
fugal action.  Or  the  coal- 
dust  may  be  obtained  from 
the  settlings  in  the  water 
tanks,  into  which  the  water 
that  has  been  used  for  wash- 
ing the  coal  is  allowed  to  run. 
In  the  most  modern  forms  of 
coal-washing  machinery,  the 
water,  after  it  has  done  its 
work  in  washing  the  coal, 
is  passed  into  tanks  provided 
for  the  purpose,  where  the 
dust  which  it  has  picked  up 


io8  ELECTRICITY   IN   MINING 

in  the  process  of  washing  gradually  settles  to  the  bottom,  the  water 
then  being  used  again;  and  the  coal-dust,  which  can  be  removed 
from  the  tanks  after  the  water  has  been  drawn  off,  and  which  is 
in  as  finely  divided  a  state  as  that  produced  by  any  crushing 
medium,  can  be  used  for  firing  boilers.  The  coal-dust  from  the 
settling  tanks  is  used  at  some  Yorkshire  collieries — simply  hand- 
fired  into  the  boiler  furnace.  The  plan  adopted  by  the  Schwartz  - 
kopff  Co.,  which  is  shown  in  Fig.  56,  is,  the  fine  dust  is  carried 
into  the  boiler  from  the  hopper  in  a  cloud,  by  the  aid  of  a  revolving 
brush  fixed  below  the  hopper,  the  brush  breaking  up  the  dust  and 
delivering  it  to  the  boiler  in  the  form  mentioned. 

The  Boiler-feed   Problem 

In  providing  power  for  mining  work,  the  boiler-feed  problem  is 
often  a  very  important  one,  quite  apart  from  the  question  of  whether 
steam  is  being  generated  simply  for  use  in  the  engines  direct,  or  for 
use  in  an  electricity  generating  station.  Though  a  great  number 
of  mines  have  to  deal  with  a  large  quantity  of  water  in  their  work- 
ings, which  has  to  be  pumped  to  the  surface  and  got  rid  of  in  some 
way,  it  rarely  happens  that  this  water  can  be  used  for  generating 
steam.  The  mere  fact  that  it  is  pumped  out  of  the  mine  presupposes 
that  it  contains  salts,  that  would  have  a  very  serious  effect  upon  the 
life  of  the  heating  surface  of  the  boiler.  Water  has  among  its  other 
properties  the  very  important  one  of  dissolving  salts  of  all  kinds, 
from  the  rocks  and  earth  it  flows  over  or  through,  and  it  also 
has  the  property  of  carrying  along  with  it  minute  particles  of  the 
rocks  and  earth  in  a  state  of  mechanical  suspension.  On  the  other 
hand,  efficient  steam  generation,  that  is  to  say,  generation  at  the 
lowest  possible  cost  in  coal  and  attendance,  requires  that  the  distance 
between  the  hot  gases  passing  through  the  boiler  flues  or  tubes 
round  them,  and  the  water  on  the  other  side  of  the  tubes  or  flues, 
shall  be  as  small  as  possible.  The  most  efficient  heating  arrange- 
ment, other  things  being  the  same,  would  be  a  very  thin  plate  of 
highly  conducting  metal,  such  as  copper,  with  a  very  thin  film  of 
water  on  one  side,  and  the  stream  of  gases  also  passing  in  a  very 
thin  layer  on  the  other  side.  If  the  water  used  in  a  boiler  contains 
salts,  these  are  deposited  upon  the  surface  of  the  tubes,  etc.,  on  the 
water  side,  and  gradually  build  up  a  thickness  of  a  substance  which 
offers  a  considerable  resistance  to  the  passage  of  the  heat  from  the 
furnace  gases  through  it.  Hence  it  is  important  that  the  purest 
water  obtainable  should  be  used.  There  is  also  another  factor  in  the 
problem.  When  water  is  being  heated  in  a  boiler,  the  most 
economical  heating  will  take  place  if  the  water  is  kept  continually 
in  circulation,  presenting  successive  new  surfaces  to  the  hot  plates  or 


THE   GENERATION   OF   ELECTRICITY  109 

tubes,  etc.     If  water  is  allowed  to  remain  at  rest  on  a  heated  surface, 
minute  globules  of  steam  or  of  heated  water  form  between  the  surface 
of  the  plate  and  the  water  beyond,  these  globules  offering  a  high 
resistance  to  the  passage  of  heat  through  them,  and  consequently  the 
boiler  tubes,  etc.,  reach  dangerous  temperatures,  while  the  heat  does 
not  reach  the  water  in  the  way  that  it  should  do.     If,  when  a  proper 
circulation  is  created  in  a  boiler,  cold  water  is  forced  into  it  at  any 
part,  the  presence  of  the  cold  water  instantly  checks  the  circulation, 
and  the  result  is  that   the  coal  consumed  is  increased.     A  little 
consideration  will  show  the  importance  of  this.     Take  the  ideal  case 
given  above  of  a  very  thin  copper  plate  of  very  large  dimensions, 
with  a  very  thin  stream  of  water  passing  over  it  in  one  direction,  and 
a  very  thin  stream  of  hot  gases  passing  under  it  in  the  opposite 
direction ;   the  economy  of  steam  generation  under  such  conditions 
would  be  the  greatest  obtainable.     Now  imagine  some  obstruction 
to  be  placed  in  the  way  of  the  stream  of  water  on  the  top  of  the 
plate.     Imagine  a  bridge  of  non-conducting  material,  so  arranged  as 
to  break  the  continuity  of  the  heating  surface,  to  cause  the  water  to 
break  its  passage  and  to  go  some  distance  out  of  its  way  over  the 
obstruction.     The  result  would  be  that  the  water  would  be  partially 
cooled  before  it  reached  the  heating  surface  again,  the  continuity  of 
the  heating  surface  being  broken,  a  much  larger  quantity  of  heat 
would  have  to  be  delivered  to  the  water  to  enable  it  to  reach  the 
boiling  temperature,  while  the  water  itself  would  not  have  received 
as  large  a  quantity  of  heat  as  it  would  have  done  under  the  ideal 
conditions   sketched.      The   obstruction    sketched  is    created  when 
water  at  a  much  lower  temperature  than  that  which  is  already  in 
the  boiler  is  pumped  into  it,  and  it  is  therefore  of  importance,  quite 
apart  from  other  economies,  that  the  feed  water  should  be  heated  as 
described,  by  the  feed-water  heater  and  the  economizer.     Further, 
the  importance  of  heating  the  feed  water  is  so  great  that  it  has  been 
shown  to  be  economical  to  employ  live  steam  from  the  boiler  itself — 
steam  that  has  been  generated  by  the  expenditure  of  heat  in  the 
boiler,  to  raise  the  temperature  of  the  feed  water  that  is  to  be  pumped 
into  the  boiler.     There  are  three  methods  of  feeding  the  boiler — by 
steam-driven  pumps,  by  electrically  driven  pumps,  and  by  injectors. 
The  steam  pump,  as  developed  by  the  Worthington  Co.  and  others, 
is  a  very  economical  apparatus.     It  consists  of  two  cylinders  fixed 
on  one  bed  plate,  each  cylinder  having  its  own  piston,  but  one 
piston  rod  connecting  the  two ;  one  is  the  steam  cylinder,  and  the 
other  the  water  cylinder.     As  the  piston  of  the  steam  cylinder  moves 
to  and  fro  it  forces  the  plunger,  or  whatever  the  arrangement  may 
be  in  the  water  cylinder,  to  and  fro,  sucking  the  water  from  the 
supply  at  one  part  of  the  stroke,  and  forcing  it  towards  the  boiler  in 
the  other  part.     In  the  Worthington  pump  the  apparatus  is  made  in 


no 


ELECTRICITY   IN   MINING 


duplicate.  Two  pump  cylinders  and  two  steam  cylinders  stand  side 
by  side,  and  they  are  rendered  automatic  by  a  valve  rod,  which  is 
common  to  the  two  pairs  of  cylinders,  and  which  by  a  very  simple 
arrangement  works  the  slide  valve  controlling  the  entrance  of  steam 
to  the  steam  cylinders.  The  valve  rod  carries  a  vibrating  arm,  which 
moves  a  rod  connected  to  the  slide  valve  also ,  the  latter  bringing  the 
slide  into  the  position  necessary  for  actuating  its  piston.  The 
Worthington  feed  pump  has  a  very  simple  arrangement  of  valves  in 
its  water  cylinder,  as  shown  in  Fig.  57.  It  has  a  long  piston 
working  to  and  fro,  and  there  are  a  number  of 
small  valves  on  the  suction  side  closed  by  springs, 
and  a  similar  number  of  small  valves  on  the 
delivery  side  also  closed  by  springs.  The  piston 
works  inside  a  ring,  which  divides  the  water 
cylinder  into  two  equal  portions,  the  pump  being 
thereby  rendered  double  acting.  As  the  piston 
recedes  from  one  half  the  suction  valves  open, 
the  delivery  valve  closing,  the  suction  valve  in 


FIG.  57. — Sectional  View  of  Worthington  Plunger  and  Ring  Pump.  The 
Steam  Cylinder  is  shown  on  the  Left,  the  Water  Cylinder  on  the  Bight, 
and  the  Vibrating  Arm  which  works  the  Steam  Admission  Valve  in  the 
Centre. 

the  other  half  closing  and  the  delivery  valve  opening.  As  the 
piston  returns  to  that  half,  the  suction  valve  in  that  half  closes, 
and  the  delivery  valve  opens,  and  so  on.  The  Worthington  boiler- 
feed  pump  may  be  seen  working  in  almost  every  kind  of  works 
where  steam  is  employed,  without  any  attention  whatever,  its  valve 
rod  and  its  pistons  working  to  and  fro,  as  explained.  It  is  made 
in  various  forms,  with  two  cylinders  taking  steam  at  the  same  pres- 
sure, and  with  four  steam  cylinders,  two  being  arranged  behind  the 
other  two,  and  the  steam  side  working  compound.  It  is  also  made 


THE   GENERATION   OF   ELECTRICITY  in 

for  working  against  pressures  up  to  300  Ibs.  per  square  inch,  and  it  is 
made  in  the  form  of  the  ram  pump,  with  simple  steam  or  compound 
steam  cylinders.  The  pumps  are  also  made  horizontal  or  vertical, 
the  horizontal  being  those  most  frequently  met  with. 

The  electrically  driven  pump  for  boiler  feed  is  nearly  always 
the  three-throw  ram  pump.  This  pump  will  be  explained  more  fully 
in  Chapter  VI.  Meanwhile  it  may  be  mentioned  that  there  are  three 
pump  cylinders,  each  having  a  piston  attached  to  a  rod,  the  three 
piston-rods  coming  out  to  three  cranks  carried  on  one  shaft,  and  to 
this  shaft  an  electric  motor  is  geared.  The  three  cranks  are  spaced 
120°  apart  on  the  crank  shaft,  and  each  ram  or  plunger,  as  it 
rises  in  its  cylinder,  opens  its  suction  valve,  the  water  entering  the 
cylinder,  and  as  it  is  forced  downwards  the  suction  valve  closes,  the 
delivery  valve  opens,  and  the  water  is  forced  into  the  boiler,  or 
wherever  it  is  required.  A  variation  of  the  three-throw  pump  is  the 
variable  stroke  three-throw  pump,  of  which  there  are  two  forms,  both 
electrically  driven,  the  object  being  to  vary  the  quantity  of  water 
delivered  to  the  boiler,  or  elsewhere,  without  varying  the  speed  at 
which  the  motor  is  running.  With  the  ordinary  arrangement  of 
the  three-throw  pump  the  stroke  is  fixed,  and  the  quantity  of  water 
pumped  depends  simply  upon  the  length  of  the  stroke  and  the  number 
of  strokes  per  minute,  multiplied,  of  course,  by  the  number  of 
pump  barrels,  so  that  variation  in  the  quantity  of  water  pumped  in 
a  given  time  can  only  be  obtained  by  varying  the  number  of  strokes 
per  minute,  that  is,  the  speed  at  which  the  pump  is  running. 
Messrs.  Mather  &  Platt  exhibited  at  the  Glasgow  Exhibition  of 
1901  a  variable  stroke  pump,  in  which  the  three  cylinders  are  arranged 
in  a  cylindrical  casting  around  a  centre,  the  pump  being  driven  by 
an  electric  motor,  and  the  stroke  of  the  pump  being  regulated  by  an 
arrangement  provided  for  the  purpose,  so  that  the  quantity  of  water 
at  each  stroke  is  varied  instead  of  the  number  of  strokes.  Messrs. 
Hayward,  Tyler  &  Co.  have  also  introduced  a  variable  stroke  three- 
throw  pump,  in  which  the  three  pump  cylinders  are  carried  in  one 
casting  in  the  usual  way,  the  three  pump  rods  coming  up  to  three 
eccentrics  on  the  crank  shaft  instead  of  to  three  cranks,  and  the 
stroke  of  the  pump  is  varied  by  means  of  the  eccentrics.  The  boiler- 
feed  water  may  be  heated  either  by  making  use  of  a  portion  of  the 
wasted  heat  of  the  gases  from  the  boiler  furnace,  or  by  using  a 
portion  of  the  heat  remaining  in  the  exhaust  steam,  or  both.  In  the 
latest  electrical  generating  stations  the  feed  water  passes  first  through 
a  steam  feed  water-heater,  and  then  through  an  economizer. 


112 


ELECTRICITY   IN   MINING 


Injectors 

The  injector  is  an  apparatus  that  is  making  its  way  more  and 
more.  Though  it  is,  strictly  speaking,  a  pump,  in  that  it  draws 
water  from  the  hot  well  or  tank,  or  wherever  the  water  supply  is  to 
come  from,  and  delivers  it  to  the  boiler,  it  operates  by  the  aid  of 


FIG.  58.— Arrangement  of  Messrs.  Holden  &  Brookes'  Injector  for 
feeding  a  Lancashire  Boiler ;  View  in  Front  of  Boiler. 

steam  alone,  and  without  any  moving  parts.  The  apparatus  consists 
of  an  arrangement,  as  shown  in  Fig.  58,  in  which  there  is  a  pipe 
connected  to  a  supply  of  steam  as  from  the  boiler,  ending  in  a  nozzle, 
arranged  so  that  the  steam  will  enter  at  a  considerable  velocity.  A 
pipe,  carrying  water,  enters  the  chamber  into  which  the  steam  from 
the  injector  delivers,  and  the  steam,  passing  onwards  through  the 


PLATE  SA. — A  Battery  of  Green's  Economizer  Tubes  in  Process  of  being  Fixed 
at  Pendlebury  Collieries.  The  Tubes  will  be  built  in,  the  Flue  shown  being 
connected  to  the  Chamber. 


PLATE  SB. — Messrs.  Royles'  Water  Softening  Plant,  fixed:"at  Dalborne 
Collieries,  Lancashire. 

[To  face  p.  112. 


THE   GENERATION   OF  ELECTRICITY  113 

passages  of  the  apparatus  into  the  boiler,  draws  the  water  after  it, 
carrying  it  with  it  into  the  boiler.  The  injector  may  be  used  with 
exhaust  steam,  though  care  must  be  taken  that  the  oil  which  comes 
over  from  the  engine  cylinder  with  the  exhaust  steam,  is  eliminated 
from  the  steam  before  it  enters  the  injector,  or  the  oil  will  be  passed 
into  the  boiler  with  the  steam.  When  the  injector  is  used,  the  feed- 
water  heater  is  sometimes  displaced,  since  the  steam  heats  the  water 
it  carries  with  it  to  a  certain  temperature.  It  is  claimed  that  an 


FIG.  59. — Arrangement  of  Messrs.  Holden  &  Brooke's  Injector  for 
feeding  a  Babcock  &  Wilcox  Boiler. 

injector,  working  with  exhaust  steam,  will  feed  a  boiler  against  a 
pressure  of  75  Ibs.  per  square  inch  with  feed  water  at  about  65°  Fahr., 
while  with  live  steam  the  feed  water  may  be  injected  into  the  boiler 
against  a  pressure  of  200  Ibs.  per  square  inch,  by  properly  proportion- 
ing the  pressure  of  the  steam  in  the  injector,  the  feed  water  in  that 
case  reaching  a  temperature  of  250°  Fahr.  Figs.  58  and  59  show 
the  connections  of  Messrs.  Holden  and  Brooke's  injector  for  feeding 
Lancashire  and  water- tube  boilers  respectively. 


Economizers 

The  Economizer  has  been  so  named  by  the  firms  who  have 
introduced  it  because  it  is  claimed  to  economize  coal,  though  it  is 
only  one  of  a  number  of  apparatus  designed  for  the  same  purpose.  It 
consists  of  a  number  of  iron  tubes  fixed  vertically  inside  a  brick 

I 


n4  ELECTRICITY  IN   MINING 

chamber,  through  which  the  hot  gases  from  the  boiler  are  led 
on  their  way  to  the  chimney,  and  the  feed  water  for  the  boiler  is 
forced  through  these  tubes  on  its  way  to  the  boiler.  A  portion  of 
the  heat  contained  in  the  gases  is  extracted  from  them  as  they  pass 
through  the  economizer,  the  heat  being  delivered  to  the  feed  water. 
The  hot  gases  which  pass  around  the  economizer  tubes  deposit  carbon 
in  the  form  of  soot  on  the  outside  of  the  tubes,  and  unless  this  is 
continually  removed,  there  is  the  same  building-up  of  resistance  to 
the  passage  of  heat  to  the  water  inside  the  tubes,  as  with  the  scale 
on  the  inside  of  the  boiler.  The  makers  of  economizers  have 
grappled  with  the  problem,  and  all  apparatus  of  the  kind  are  fitted 
with  scrapers,  arranged  to  clasp  the  outsides  of  the  tubes,  and  to  be 
kept  continually  in  motion,  removing  the  deposit  of  soot,  the  soot 
being  allowed  to  fall  into  a  pit  below,  and 
fiot  WAi£&an»iSHlUT  removed  when  convenient.  The  scrapers  are 
worked  from  above  by  a  mechanical  arrange- 
ment devised  for  the  purpose,  consisting  of  a 
long  girder  supported  at  each  end,  and  carrying 
pulleys,  over  which  chains  pass,  the  chains 
having  scrapers  at  each  end.  The  chains  are 
kept  moving  up  and  down,  pulling  the  scrapers 
up  and  down,  by  any  convenient  source  of 
power,  such  as  a  small  engine,  and  more  fre- 
quently, when  electricity  is  generated  on  the 
ground,  by  an  electric  motor.  There  are  a 
number  of  forms  of  economizers,  the  differences 
between  them  being  in  the  forms  of  the  tubes, 
and  in  the  arrangement  of  the  scrapers  and 
methods  of  driving  them.  In  all  forms  it  is 
endeavoured  to  arrange  that  a  thin  stream  of 
water  shall  be  passing  in  the  tubes  at  a  com- 
paratively high  velocity,  separated  only  by  a 
small  thickness  of  metal  from  the  hot  gases. 
Plate  2D  shows  Carter's  economizer,  and 

Plate  SA  a  battery  of  Green's  being  fixed  at 
FIG.    60.— One    Arrange-       Pnlliprv 
ment    of    Feed  -  water  a  C0il  er7- 
Heater.      The     Steam 
passes      through      the 
Pipes,    the    Water    in  -^        _ 

the  space  surrounding  reccl-water  Heaters 

them. 

The  feed-water  heater  consists  of  a  closed 

vessel  filled  with  tubes,  and  the  arrangements  are  different  in 
apparatus  by  different  makers.  In  some  forms  the  steam  passes 
through  the  tubes,  and  the  water  in  the  space  surrounding  them. 
In  other  forms  the  water  passes  through  the  tubes,  and  the  steam, 


THE   GENERATION   OF   ELECTRICITY  115 

in  the  space  surrounding  them.      In  either  case  the  steam  is  made 


FIG.  61.— Boyle's  Feed-water  Heater.          FIG.  62.— Vertical  Transverse  Section 

of  Messrs.  Boyle's  Feed-water 
Heater,  showing  the  specially 
formed  Tubes  employed. 

to  deliver  as  much  as  possible  of  its  heat  to  the  water.     Figs.  60, 
61,  62  show  two  forms  of  feed- water  heaters. 


n6  ELECTRICITY  IN   MINING 


Water  Softeners 

Another  important  source  of  economy  in  connection  with  boiler 
work,  in  a  great  many  cases,  is  the  water  softener.  The  only  water 
available  for  raising  steam  very  often  contains  salts  of  lime  and 
magnesia,  and  other  substances.  The  salts  commonly  present  are  the 
carbonates  of  lime  and  magnesia,  which  give  rise  to  what  is  called 
temporary  hardness.  The  sulphates  of  lime  and  magnesia,  and  other 
elements,  are  also  sometimes  present,  giving  rise  to  what  is  called 
permanent  hardness.  The  peculiarity  of  the  carbonates  is  that  they 
are  usually  present  in  water  as  bicarbonates,  that  is,  with  two  por- 
tions of  carbonic  acid  in  combination  with  the  metal.  The  bicarbonates 
are  soluble  in  water  at  ordinary  temperatures,  but  the  carbonates  are 
insoluble.  Hence,  when  the  feed  water  contains  bicarbonates,  they 
pass  into  the  boiler,  dissolved  in  the  water ;  but  when  the  water  is 
heated,  one  portion  of  carbonic  acid  is  driven  off,  the  carbonate  is 
precipitated  upon  the  surface  of  the  boiler  on  the  water  side,  and  a 
scale  is  gradually  built  up  which  has  a  considerable  thermal  resist- 
ance, besides  reducing  the  available  water  space.  The  sulphates  which 
form  the  permanent  hardness  are  not  driven  off  by  heat,  but  they  are 
also  deposited  upon  the  water  surface  of  the  boiler,  largely  owing  to 
electrolytic  action,  and  the  same  result  follows.  The  efficiency  of  a 
boiler  may  be  very  considerably  reduced  by  the  presence  of  a  certain 
thickness  of  scale.  All  the  water  softeners  on  the  market  are  on 
certain  general  lines.  In  nearly  all  of  them  heat  is  applied  to  the 
feed  water  before  it  is  allowed  to  enter  the  boiler,  for  the  purpose  of 
driving  off  the  carbonic  acid,  thereby  producing  the  insoluble  car- 
bonates, and  these  are  precipitated  by  the  addition  of  a  certain 
quantity  of  lime.  The  permanently  hard  salts  are  also  deposited 
by  the  addition  of  lime  and  soda.  In  addition  to  the  above  arrange- 
ments, nearly  all  the  water  softeners  include  filters,  which  extract  the 
insoluble  substances  that  have  been  produced  by  the  action  of  the 
chemical  reagents.  The  makers  of  water- softening  apparatus  have 
turned  their  attention  principally  to  making  their  processes  automatic, 
and  their  apparatus  contain  various  devices  by  which  a  certain 
quantity  of  the  reagents  are  added  to  a  certain  quantity  of  water 
periodically,  a  rather  favourite  form  being  a  tumbling  arrangement, 
which,  when  filled  with  water,  receives  a  definite  charge  of  chemicals, 
and  then  empties  itself  into  the  next  portion  of  the  apparatus. 
Plate  SB  shows  Boyle's  water  softener  fixed  at  a  colliery. 


THE   GENERATION   OF   ELECTRICITY 


117 


Grease  Extractors 

Another  apparatus,  also  tending  to  economy  in  the  same  way  as 
does  the  water  softener,  and  sometimes  in  combination  with  it,  is  the 
grease  extractor,  or  the  oil  separator,  as  it  is  frequently  called.  It  is 
necessary  to  use  oil  in  the  steam  cylinder  for  lubricating  the  piston, 
and  at  every  stroke  a  certain  small  quantity  of  the  oil  is  carried  over 
into  the  exhaust  in  a  finely  divided 
state,  and  a  portion  of  it  is 
frequently  deposited  upon  the 
condenser  tubes,  where  surface 
condensers  are  employed,  but  some 
of  it  finds  its  way  back  into  the 
boiler,  and  is  one  of  the  most 
frequent  causes  of  the  formation 
of  scale.  There  are  almost  in- 
numerable oil  separators  on  the 
market,  all  of  them  working  very 
much  on  the  same  lines.  The 
steam  in  its  passage  to  the  con- 
denser is  passed  through  a  vessel 
in  which  it  is  given  a  whirling 
motion,  and  in  which  there  are  a 
number  of  baffles,  the  object  being 
to  stop  the  passage  of  the  minute 
globules  of  oil,  while  the  steam 
passes  on  its  way,  the  oil  draining 
afterwards  to  the  bottom  of  the 
vessel,  and  being  removed  by  a 
pump,  and  used  over  and  over 
again  after  filtration.  The  objec- 
tion to  this  method  of  extracting 
the  oil  is  that  the  pump  which 
removes  the  oil,  can  only  do  so 

by  creating  a  higher  vacuum  in  FIQ  63._one  Form  of  Wells' Filtering 
the  oil  separator,  than  the  air  pump  Apparatus  for  Waste  Oil. 

is  producing  in  the  condenser.    On 

the  other  hand,  it  is  a  distinct  advantage  to  remove  the  oil  from 
the  steam  before  it  passes  into  the  condenser,  because  the  efficiency 
of  the  condenser  will  be  distinctly  reduced  by  the  deposit  of  oil  upon 
its  pipes,  and  this  is  a  not  infrequent  source  of  trouble  with  surface 
condensers.  If  the  oil  is  not  removed  before  the  steam  passes  into 
the  condenser,  an  oil  filter  is  interposed  in  the  path  of  the  feed  water 
before  it  enters  the  boiler.  The  oil  can  be  used  over  again,  if  filtered. 
Fig.  63  shows  one  of  Wells'  apparatus  designed  for  the  purpose. 


n8  ELECTRICITY   IN   MINING 


Coal  Conveyers 

Another  apparatus  which  contributes  to  economy,  where  the  station 
is  of  sufficient  size  to  warrant  its  adoption,  is  the  coal-handling 
plant.  In  large  stations,  which  are  arranged,  if  possible,  on  the 
bank  of  a  river,  or  on  a  railway  siding,  and  to  which  the  coal  is 
brought  either  in  barges  or  in  trucks,  there  is  now  a  complete 
apparatus  for  unloading  the  barges  and  trucks,  carrying  the  coal  to 
coal  bunkers,  which  are  usually  placed  at  the  top  of  the  building, 
weighing  it  on  its  way  to  the  bunkers,  and  carrying  it  from  the 
bunkers  to  the  hoppers  over  the  boiler  furnaces.  The  first  arrange- 
ment consists  of  a  crane,  to  which  is  attached  what  is  called  a  "  grab." 
The  grab  is  practically  a  coal  box  split  into  two  halves,  each  half 
being  hinged  separately,  and  held  from  the  end  of  the  chain  worked 
by  the  crane.  When  the  weight  of  the  grab  and  the  coal  it  contains 
is  on  the  chain,  the  two  halves  of  the  grab  close  up  together,  forming 
practically  a  box  with  the  cover  in  two  halves  held  loosely  together. 
When  the  grab  is  lowered  into  the  barge  or  truck,  it  opens,  the  two 
halves  separating  out,  and  sinking  by  their  weight  into  the  mass  of 
coal,  which  usually  has  a  large  proportion  of  small,  in  a  modern 
power  station ;  and  when  tension  is  put  on  the  chain,  the  two  halves 
move  towards  each  other,  enclosing  the  coal  they  have  grabbed.  The 
whole  thing  is  run  up  to  the  head  of  the  crane,  swung  round  over 
usually  a  large  receiver  or  hopper,  to  which  a  weighing  machine  is 
attached.  Thence  the  coal  passes  by  a  bucket  conveyer  to  the  top  of 
the  building,  where  it  is  delivered  to  a  band  conveyer,  which  carries 
it  to  its  temporary  destination.  The  bucket  conveyer  consists  of  an 
endless  chain  fixed  at  an  angle  with  the  vertical,  depending  upon  the 
position  to  which  it  is  required  to  carry  the  substance  being  handled, 
and  having  attached  to  its  links,  buckets  or  scoops,  which  pass 
round,  and  over  rollers  at  the  top  and  bottom.  The  bottom  of  the 
conveyer  is  so  arranged  that  the  bucket  which  is  emerging  from  the 
off-side,  scoops  a  bucketful  out  of  the  coal  lying  in  a  mass  ready  for  it. 
The  bucketful  is  carried  to  the  top  of  the  conveyer,  and  is  there 
tipped  over  on  to  the  next  apparatus  that  is  to  receive  it.  The  band 
conveyer  is  simply  a  wide  belt  running  on  rollers,  the  coal  being 
tipped  on  to  it  in  such  a  manner  that  it  spreads  out  along  the  belt  in 
a  thin  stream,  and  is  carried  by  it  to  the  bunker  that  is  being  filled. 
At  the  bunker  the  belt  is  suddenly  given  a  sharp  turn  by  means  of 
special  pulleys,  interposed  in  its  path,  and  the  coal  is  shot  forward 
into  its  place.  Another  form  of  conveyer  that  is  often  used  where 
very  fine  coal  is  employed  is  the  "  archimedean "  screw.  The 
archimedean  screw  or  traveller  is  a  screw  with  a  very  wide  blade  and 
a  very  coarse  pitch.  It  runs  in  a  trough,  which  may  be  open  at  the 


THE   GENERATION   OF   ELECTRICITY  119 

top,  the  coal  or  other  substance  to  be  conveyed  being  delivered  to 
the  trough  at  one  end.  The  blade  of  the  screw  engages  with  a  small 
portion  of  the  coal  as  the  screw  revolves,  and  it  carries  that  portion 
forward  in  its  revolution,  each  turn  of  the  screw  engaging  with 
some  coal,  the  coal  remaining  at  the  bottom  of  the  trough,  but 
being  continually  moved  forward,  and  is  delivered  to  whatever  is  to 
receive  it  at  the  other  end,  the  last  turn  of  the  screw  pushing  it  out. 
Conveyers  are  also  employed  for  delivering  the  coal  to  the  hoppers 
over  the  furnaces,  the  conveyer  being  loaded  at  one  end  or  at  any 
convenient  portion  from  the  bunker  overhead,  the  coal  usually 
passing  through  a  weighing  machine  on  its  way  to  the  boiler  furnace. 
The  same  conveyer,  or  another  specially  arranged  for  the  purpose,  is 
sometimes  used  for  carrying  the  ashes  away  from  the  ashpit,  and 
delivering  them  either  on  to  small  trucks  arranged  to  receive  them  at 
the  end  of  the  boiler  range,  or  to  any  convenient  receptacle. 

Steam,  generated  in  any  of  the  boilers  described,  may  be  used  in 
either  reciprocating  engines,  or  turbines,  to  drive  electric  generators. 


Reciprocating  Steam  Engines 

Reciprocating  steam  engines  are  broadly  divided  into  two  groups, 
known  respectively  as  "  high-speed  "  and  "  low-speed  "  engines.  The 
terms  are  really  very  misleading,  as  the  piston  speed,  which  is  the 
determining  factor  in  any  engine,  is  the  same  in  the  so-called  slow-speed 
engines  as  in  the  high-speed  engines.  In  addition  to  this,  also,  in  the 
so-called  slow-speed  engines  there  are  larger  and  heavier  masses  of 
metal  in  motion  than  in  the  high-speed  engines,  and  it  is  perhaps  not 
surprising  that  the  so-called  high-speed  engine  is  gradually  displacing 
the  so-called  low-speed  engine  for  a  great  deal  of  the  new  wo»k  that  is 
being  put  down.  The  terms  "  quick- revolving  "  and  "  slow-revolving  " 
were  substituted  a  short  while  since  for  the  terms  "  high-speed  "  and 
"  low-speed,"  and  those  terms  very  much  more  accurately  express  what 
actually  takes  place.  The  so-called  high-speed  engine  runs  at  300 
and  400  revolutions  per  minute,  while  the  so-called  slow-speed 
engine  runs  at  anywhere  from  50  up  to  150  revolutions  per  minute. 
Not  many  years  ago  the  limit  was  from  20  to  100  revolutions  for 
slow  speeds.  The  same  power  is  obtained  from  a  large  piston  with  a 
long  stroke,  and  making  only  a  few  revolutions  per  minute,  as  from 
a  smaller  piston  with  a  shorter  stroke  making  a  larger  number  of 
revolutions  per  minute.  The  high-speed  engine  was  for  a  long  time 
looked  at  with  considerable  doubt  by  practical  engineers.  They  feared 
that  the  engine  would  knock  itself  to  pieces,  and  a  great  many  of  the 
earlier  high-speed  engines  did  so.  On  the  other  hand,  the  large 
slowly  revolving  low-speed^engines  rarely  gave  any  trouble,  and  were, 


120  ELECTRICITY   IN    MINING 

and  are  still,  very  economical  in  steam,  when  worked  with  Corliss 
valves,  and  with  the  other  modern  apparatus  that  has  been  brought 
into  service  for  increasing  the  steam  economy.  The  advance  in  the 
construction  of  high-speed  engines  is  very  largely  due  to  the 
researches  of  the  late  Mr.  Willans,  of  the  firm  of  Willans  &  Bobin- 
son.  It  may  almost  be  said  that  the  Willans  engine  was  the  first 
quickly-revolving  engine  that  achieved  practical  success.  But  the 
Willans  engine  overcame  the  difficulties  of  lubrication,  which  were 
the  great  stumbling-block  to  the  early  inventors  of  high-speed  engines, 
by  a  new  departure,  and  by  practically  sacrificing  half  the  work 
the  engine  can  be  made  to  perform.  In  the  Willans  engine  steam 
is  only  taken  on  one  side  of  the  piston,  and  the  crankshaft  revolves 
in  a  bath  of  oil  and  water,  the  whole  apparatus  being  enclosed  and  the 
valves  being  carried  in  the  piston  rod  itself.  The  arrangement  makes 
a  very  compact  engine,  especially  where,  as  is  usually  arranged  with 
compound  and  occasionally  with  triple-expansion  Willans  engines, 
the  high-pressure,  intermediate,  and  low-pressure  cylinders  are  fixed 
vertically  one  above  the  other,  one  piston  rod  carrying  the  whole  of 
the  pistons,  and,  of  course,  the  whole  of  the  steam  valves.  Forced 
lubrication,  as  applied  in  the  Belliss  and  other  engines,  under  which 
arrangement  a  constant  supply  of  oil  is  forced  into  the  bearings  of 
all  the  moving  parts  of  the  engine,  enabled  the  high-speed  engine  to 
be  worked  with  steam  entering  on  both  sides  of  the  piston,  and  it 
is  now  claimed  that  the  Belliss  and  similar  engines  are  more 
economical  in  steam,  and  quite  as  reliable  as  the  Willans  engine. 

There  is  little  to  be  said  about  the  slowly-revolving  engines, 
except  that  the  substitution  of  the  Corliss  valves  for  the  old  pattern 
slide  valves  has  enormously  increased  their  economy  in  steam.  The 
Corliss  valve  consists  of  a  cylinder  of  metal,  in  which  the  valve 
ports  are  cast,  which  revolves  inside  a  cylindrical  space  in  the  engine 
casting,  in  which  are  other  ports,  the  motion  of  the  crankshaft 
bringing  the  different  ports  opposite  each  other,  for  the  purpose  of 
the  admission  of  steam  to  the  engine  cylinder,  the  reverse  motion 
cutting  it  off  in  the  same  manner  as  the  motion  of  the  slide  valve 
along  the  face  of  the  cylinder,  performs  the  same  operations. 


Working  Steam  Engines  expansively 

Steam  engines  are  classed  under  the  headings,  simple  engines, 
compound  engines,  triple  and  quadruple  expansion  engines.  In  the 
simple  engine  there  is  one  cylinder,  and  the  steam  enters  behind  the 
piston,  pushes  it  to  the  end  of  the  stroke,  and  passes  out  to  the  atmo- 
sphere or  to  the  condenser.  The  simple  engine  may  consist  of  two 
cylinders  coupled  together,  with  a  flywheel  or  similar  arrangement 


THE   GENERATION   OF  ELECTRICITY  121 

between  them,  each  cylinder  taking  steam  at  the  full  boiler  or  throttle 
valve  pressure,  and  the  steam  passing  from  each  cylinder  to  the 
atmosphere  or  to  the  condenser  independently.  In  the  compound 
engine  there  are  two  cylinders,  one  of  which  has  a  larger  piston  than 
the  other.  The  cylinders  are  termed  "  high  "  and  low  "  pressure,"  and 
the  steam  enters  the  high-pressure  cylinder,  passes  from  the  exhaust 
of  the  high-pressure  cylinder  usually  to  a  receiver,  thence  to  the  low- 
pressure  cylinder,  and  thence  to  the  atmosphere  or  the  condenser. 
There  is  a  special  form  of  compound  engine  made  by  Messrs.  Belliss 
&  Morcom,  and  others,  in  which,  the  two  cylinders  being  placed  so 
that  their  cranks  are  180°  apart,  no  receiver  is  necessary,  since  the 
low-pressure  cylinder  will  be  taking  steam  at  the  same  time  as  the 
high-pressure  cylinder  is  exhausting.  In  the  triple-expansion  engine 
there  are  three  cylinders,  sometimes  four,  named  respectively  the 
high-pressure  or  H.P.,  the  intermediate  or  I.P.,  and  the  low-pressure 
or  L.P.  Where  there  are  four  cylinders,  the  low-pressure  cylinder  is 
divided  into  two,  and  the  object  is  to  obtain  a  more  even  turning 
moment  on  the  crankshaft  and  a  better  distribution  of  the  steam. 
The  steam  passes  first  into  the  high-pressure  cylinder,  thence  to  a 
receiver,  thence  to  the  intermediate  cylinder,  thence  to  a  receiver, 
thence  to  the  low-pressure  cylinder  or  cylinders,  and  from  there  to 
the  atmosphere  or  condenser.  In  the  quadruple-expansion  engine 
there  are  four,  and  sometimes  five,  cylinders.  Where  there  are  five 
cylinders,  the  low-pressure  cylinder  is  again  divided  into  two,  and 
the  steam  passes  in  succession  through  the  high-pressure,  the  first 
intermediate,  the  second  intermediate,  and  the  low-pressure,  thence 
to  the  atmosphere  or  condenser.  Quadruple-expansion  engines  are 
not  often  seen  on  shore,  but  they  are  employed  largely  in  some  of 
the  big  ocean  liners.  Triple-expansion  engines,  however,  are  becoming 
more  and  more  common  as  they  are  better  understood  and  their  manu- 
facture is  improved.  The  great  object  of  the  additional  cylinders  is 
the  economical  use  of  the  higher  steam  pressures  that  are  employed 
in  modern  steam  plant.  The  major  portion  of  the  heat  employed  in 
the  generation  of  steam  is  taken  up  in  converting  the  water  at  a 
certain  temperature,  into  steam  at  the  same  temperature.  With 
steam  at  atmospheric  pressure  and  at  212°  temperature,  966  units 
are  absorbed  per  pound  by  the  latent  heat  of  the  steam.  The  specific 
heat  of  steam  being  only  0*4,  it  is  easily  understood  that  the  heat 
given  to  the  steam  after  its  generation  is  very  much  more  usefully 
employed  than  that  given  to  the  water  to  convert  it  into  steam; 
but  this  is  only  on  the  condition  that  the  whole,  or  a  large  portion 
of  the  energy  in  the  steam,  can  be  employed  in  the  engines.  A  great 
deal  of  economy  in  steam  consumption  is  now  attained,  even  in 
simple  engines,  by  only  admitting  steam  to  the  cylinder  during  a 
small  portion  of  the  stroke.  In  the  earlier  forms  of  reciprocating 


122  ELECTRICITY  IN   MINING 

engines,  and  even  in  some  forms  of  engines  to  be  seen  at  the  present 
day,  the  work  done  by  the  steam  upon  the  piston  is  in  the  nature  of 
a  continuous  push.  Where  the  steam  is  admitted  behind  the  piston 
for  the  whole  of  the  stroke,  and  where  the  boiler  is  generating  steam 
as  fast  as  it  is  used  by  the  engine,  the  steam  simply  pushes  the 
piston  to  the  end  of  the  stroke,  the  push  being  taken  right  from  the 
boiler.  But  this  method  is  exceedingly  wasteful  in  steam,  and  there 
are  records  of  small  steam  engines,  steam  pumps,  and  other  apparatus 
using  as  much  as  250  Ibs.  of  steam  per  indicated  horse-power,  where 
the  latest  type  of  reciprocating  engine  working  expansively  only 
uses  12  Ibs.,  or  thereabouts.  Some  thirty  or  more  years  ago  the  first 
steps  in  the  direction  of  the  economical  use  of  steam  were  taken  by 
setting  the  slide  valves  in  the  majority  of  engines  to  cut  off  at  half 
stroke ;  that  is  to  say,  when  the  piston  had  travelled  half  its  way 
through  the  cylinder,  the  entry  of  steam  behind  it  was  stopped,  and 
the  remainder  of  the  passage  of  the  piston  was  obtained  by  the 
expansion  of  the  steam  already  in  the  cylinder,  the  pressure  of  the 
steam  gradually  falling  as  the  piston  moved  forward.  It  was  evident 
that  this  method  could  be  extended,  and  in  some  types  of  simple 
engines  the  steam  is  cut  off  as  early  as  one-tenth  of  the  stroke,  the 
remainder  of  the  work  being  done  by  the  expansion  of  the  steam 
itself,  and  it  is  claimed,  and  apparently  with  justice,  that  these 
engines  would,  combined  with  condensers,  work  with  fair  economy. 
In  the  compound,  triple,  and  quadruple  expansion  engines,  the 
steam  is  made  to  work  expansively  in  each  cylinder  as  well  as  by 
passing  through  the  cylinders  in  succession;  that  is  to  say,  the 
entrance  of  the  steam  is  cut  off  at  a  certain  portion  of  the  stroke  in 
each  cylinder,  and  in  each  cylinder  the  remainder  of  the  work  upon 
the  piston  is  performed  by  the  expansion  of  the  steam.  It  will  be 
understood  that  while  the  expansive  working  of  the  steam  in  a  single 
cylinder,  and  in  each  of  the  cylinders  of  compound  and  triple  expan- 
sion engines,  leads  to  economy  of  steam  consumption,  and  therefore  of 
coal,  it  also  means  that  a  smaller  amount  of  work  is  performed  by 
each  individual  cylinder.  Perhaps  a  few  figures  will  illustrate  the 
point  more  clearly.  While  the  steam  is  entering  the  cylinder  behind 
the  piston  its  pressure  is  the  same  as  that  of  the  steam  chest,  less  any 
pressure  that  it  may  be  deprived  of  by  the  action  of  the  governor. 
After  the  entry  valve  is  closed,  the  pressure  behind  the  piston  is 
continually  falling  until  the  end  of  the  stroke,  and  the  pressure  that 
must  be  employed  for  calculation  of  the  actual  indicated  horse  power 
generated  by  the  piston,  is  the  mean  of  all  the  successive  pressures, 
from  the  moment  the  steam  commences  to  enter  the  cylinder.  Taking 
what  is  now  very  common,  150  Ibs.  initial  pressure,  absolute,  at  the 
boiler,  or  approximately  135  Ibs.  gauge  pressure ;  with  the  steam  cut 
off  at  three-quarter  stroke,  the  mean  pressure  behind  the  piston  is 


THE   GENERATION    OF   ELECTRICITY 


123 


i24  ELECTRICITY  IN   MINING 

144'8  Ibs.  per  square  inch ;  with  steam  cut  off  at  half-stroke  the 
mean  pressure  is  126'9  Ibs. ;  with  steam  cut  off  at  quarter-stroke  it 
is  reduced  to  83'5  Ibs. ;  at  one-sixth  stroke  to  69'8  Ibs. ;  and  at  one- 
tenth-stroke  it  is  49 '5  Ibs.  That  is  to  say,  with  a  cut-off  at  one- 
tenth  of  the  stroke,  the  mean  pressure  operating  to  drive  the  piston 
forward  in  any  particular  cylinder,  with  an  initial  gauge  pressure  of 
135  Ibs.  per  square  inch,  is  a  little  over  one-third  of  that  when  the 
steam  is  allowed  to  pass  into  the  cylinder  for  three-quarters  of  the 
stroke,  or  the  engine  would  only  perform  a  little  less  than  one-third 
the  work  at  one-tenth  cut-off  that  it  would  at  three-quarters  cut-off. 
In  addition  to  this,  the  mean  effective  pressure,  the  actual  force 
available  for  driving  the  piston  forward,  is  the  mean  pressure  behind 
the  piston,  as  explained  above,  less  the  mean  pressure  of  the  steam 
which  is  in  front  of  the  piston,  the  steam  which  remains  in  the 
cylinder  after  the  piston  has  completed  its  stroke  in  one  direction, 
and  which  the  piston  has  to  drive  out  of  the  cylinder  on  the  return 
stroke. 

With  compound,  triple,  and  quadruple  expansion  engines  it  is 
always  arranged,  as  will  easily  be  understood,  that  the  work  done  on 
the  piston  of  each  cylinder,  or  pair  of  cylinders,  is  equal ;  that  is  to 
say,  the  work  done  in  the  single  high-pressure  cylinder  in  turning 
the  crankshaft  is  equal  to  that  done  in  the  low-pressure  cylinders. 
Where  the  low-pressure  cylinder  is  divided  into  two,  the  combined 
work  of  the  two  cylinders  is  equal  to  that  in  the  intermediate  cylinder 
and  to  that  in  the  high-pressure  cylinder.  This  leads,  it  will  be  seen, 
to  the  cylinders  themselves  having  different  areas,  the  high-pressure 
being,  of  course,  the  smaller,  and  the  sizes  increasing  as  the  steam 
pressure  decreases.  Further,  in  the  matter  of  the  government  of  the 
engine,  it  should  be  arranged  that  the  mean  effective  pressure  in  each 
cylinder  is  such  that  the  total  work  in  horse-power  in  each  cylinder, 
that  is  to  say,  the  product  of  the  mean  effective  steam  pressure,  multi- 
plied by  the  area  of  the  piston,  is  the  same  for  H.P.,  I.P.,  and  L.P. 
There  are  two  methods  of  arranging  the  government  of  compound  or 
triple-expansion  engines.  In  one  method  the  governor  controls  the 
cut-off  in  each  cylinder  in  such  a  manner  that  the  horse-power  in  each 
is  practically  the  same ;  in  the  other  method  the  cut-off  of  the  low- 
pressure  cylinder  is  fixed,  the  governor  merely  controlling  the  cut-off 
in  the  high-pressure  and  intermediate.  Figs.  64  and  65  show 
sections  of  Belliss'  compound  and  triple  expansion  engines.  Plate  4A 
shows  a  Belliss'  triple  expansion  engine  coupled  to  a  three-phase 
generator. 


THE   GENERATION   OF   ELECTRICITY 


126  ELECTRICITY   IN   MINING 


The  Government  of  Steam   Engines 

There  are  two  methods  of  governing  steam  engines,  known 
respectively  as  throttle  governing,  and  expansion  governing.  The 
object  of  the  governor  in  both  cases  is  to  proportion  the  consumption 
of  steam  to  the  work  the  engine  is  performing,  cutting  off  the  supply 
if  the  load  decreases,  and  vice  versa.  Both  methods  operate  by  means 
of  the  well-known  centrifugal  governor  with  revolving  balls,  these 
acting  either  upon  the  entry  valve  to  the  steam  chest,  or  on  the  entry 
valve  to  the  cylinder  itself.  The  throttle  governor  acts  upon  the 
entry  valve  to  the  steam  chest,  and  is  practically  the  equivalent  of  a 
stop  valve  operated  by  hand  by  a  careful  attendant,  who  is  instantly 
posted  as  to  changes  of  load.  It  operates  by  the  variation  in  the 
speed  of  the  crankshaft  of  the  engine  produced  by  a  change  of  load. 
When  the  engine  is  running,  the  governor  balls  fly  out  in  opposition 
to  a  spring  or  weight,  sometimes  to  a  combination  of  the  two.  When 
the  engine  is  running  at  its  proper  speed  with  any  load,  the  valve 
assumes  a  position  which  allows  sufficient  steam  to  enter  to  perform 
the  work  in  front  of  the  engine.  If  the  load  increases,  the  engine 
slightly  slows  down.  The  reduction  of  speed  of  the  crankshaft  is 
followed  by  a  reduction  of  the  speed  of  the  governor  balls,  the 
governor  being  driven  by  a  strap  or  gearing  from  the  crankshaft, 
and  occasionally  fixed  on  the  end  of  the  crankshaft.  The  governor 
balls  lose  a  portion  of  their  centrifugal  force,  move  slightly  towards 
the  centre,  the  spring  or  weight  opposing  them  then  slightly  opens 
the  valve,  an  increased  quantity  of  steam  enters,  and  the  engine 
recovers  its  normal  speed.  The  expansion  governor  operates  in 
exactly  the  same  manner,  but  in  place  of  acting  upon  the  stop  valve 
or  its  equivalent,  it  acts  directly  upon  the  slide  or  other  valve  con- 
trolling the  entry  of  steam  to  the  cylinder,  altering  the  period  during 
which  the  valve  is  open.  Thus,  if  the  engine  receives  an  increased 
load,  the  expansion  governor  increases  the  proportion  of  the  stroke 
before  the  steam  is  cut  off,  this,  of  course,  increasing  the  mean 
pressure  behind  the  piston  and  the  available  power.  The  expansion 
governor  is  now  almost  universal,  though  the  throttle  governor  is 
still  occasionally  to  be  seen,  and  some  engineers  prefer  it. 


Difficulties  in  the  Way  of  working  expansively 

While  expansive  working  of  steam  brings  very  considerable 
economies,  and  has  enabled  the  quantity  of  coal  consumed  per 
indicated  H.P.  to  be  reduced  from  the  neighbourhood  of  10  Ibs.  to 
1£  Ibs.,  the  usual  crop  of  difficulties  has  arisen  in  its  path,  the  first, 


THE   GENERATION   OF   ELECTRICITY  127 

and  perhaps  the  most  important,  being  the  trouble  with  the  condensa- 
tion of  steam  in  the  cylinders.  When  steam  is  generated  in  a  boiler, 
there  is  a  mechanical  action  going  on  at  the  same  time  as  the  con- 
version of  water  into  steam  is  taking  place,  minute  globules  of  water 
being  carried  over  with  the  steam  from  the  boiler  into  the  steam 
pipes,  steam  chests,  etc.  These  minute  globules  are  of  the  nature  of 
the  vapour  that  we  are  familiar  with  in  the  case  of  fogs  and  mists. 
They  are  not  steam.  They  are  possessed  of  a  certain  quantity  of 
heat,  but  not  sufficient  to  enable  them  to  maintain  the  condition  of 
misty  globules,  in  the  face  of  a  lowered  temperature  in  their  sur- 
roundings. They  meet  this  lowered  temperature  on  entering  the 
cylinder  of  an  engine  that  is  working  very  expansively.  Taking  the 
case  given  above  of  steam  at  an  absolute  pressure  of  150  Ibs.  per 
square  inch,  and  assuming,  for  simplicity,  that  the  steam  is  at  that 
pressure  on  entering  the  cylinder,  its  temperature  is  358°  Fahr. 
Assuming  that  the  steam  is  cut  off  at  a  very  early  part  of  the  stroke, 
and  that  it  is  expanded  down  to  a  few  pounds  above  atmospheric  pres- 
sure, its  temperature  at  the  end  of  the  stroke  will  be  in  the  neigh- 
bourhood of  225°  to  230°  Fahr. ;  that  is  to  say,  there  will  be  a 
difference  approximately  of  130°  Fahr.  between  the  temperature  of 
the  steam  on  its  entrance  to  the  cylinder  and  on  its  exit.  The 
cylinder  walls,  the  piston,  etc.,  follow  these  changes  of  temperature 
to  a  certain  extent,  with  the  result  that  at  the  end  of  the  stroke,  and 
at  the  commencement  of  the  succeeding  stroke,  the  temperature  of 
the  cylinder  space  into  which  the  live  steam  enters  may  be  a  great 
many  degrees  below  that  of  the  entering  steam.  The  first  result  of 
this  is  the  condensation  upon  the  cylinder  walls,  piston,  etc.,  of  the 
vapour  of  minute  water  globules  that  have  come  over  from  the  boiler 
with  the  steam,  and  this  deposited  water  is  re-evaporated  at  a  later 
period  of  the  stroke  by  the  absorption  of  heat  from  the  cylinder 
walls,  etc.,  the  result  being  that  the  steam  itself  is  possessed  of  a 
smaller  quantity  of  energy,  and  the  mean  pressure  available  for 
driving  the  piston  is  smaller  than  would  be  the  case  if  the  cylinder 
walls  had  remained  at  the  same  temperature  as  the  entering  steam. 
The  above  case  has  been  taken  by  the  writer  as  an  illustration 
because  it  shows  the  matter  so  clearly ;  but  the  case  of  the  simple 
engine  working  with  a  very  high  ratio  of  expansion  is  rather  excep- 
tional, and  the  above  is  one  of  the  reasons  that  have  led  to  the 
development  of  the  compound,  triple,  and  quadruple  expansion 
engines.  In  the  cylinders  of  compound,  triple,  and  quadruple 
expansion  engines,  however,  the  same  phenomena  occur,  though  to  a 
smaller  degree,  because  the  range  through  which  the  steam  pressure 
and  the  steam  temperature  pass,  is  divided  up  between  the  successive 
cylinders,  and  the  changes  of  temperature  in  each  cylinder  are  not  as 
great  as  where  the  whole  of  the  expansion  takes  place  in  one  cylinder. 


i28  ELECTRICITY  IN   MINING 


Overcoming  Condensation  Troubles 

There  are  two  principal  methods  employed  for  overcoming  the 
troubles  and  the  waste  due  to  the  condensation  of  steam  in  the  engine 
cylinders,  viz.  by  jacketing  the  steam  cylinders,  and  by  superheating  the 
steam.  In  the  first  method  a  jacket,  similar  to  that  fitted  to  gas  engines, 
is  fitted  to  the  engine  cylinders,  and  the  steam  is  made  to  pass  through 
this  jacket  on  its  way  to  the  entry  valve.  In  some  cases  the  steam, 
jacket  has  been  applied  to  all  the  cylinders  of  compound  and  triple 
expansion  engines,  and  it  has  also  been  applied  as  a  reheater  to  the 
receivers  between  the  different  cylinders.  The  economy  of  the  steam 
jacket  would  appear  to  depend  very  much  upon  the  number  of 
expansions,  as  it  is  usually  expressed ;  that  is  to  say,  upon  the 
period  at  which  the  entry  of  steam  is  cut  off.  If  the  steam  is  cut  off 
early,  so  that  there  is  a  wide  difference  of  temperature  between  the 
steam  entering  at  the  commencement  of  the  stroke  and  that  leaving 
the  cylinder  on  the  return  stroke,  steam  jacketing  should  be  of 
service,  and  it  is  stated  that  economies  of  as  much  as  25  per  cent. 
have  been  obtained  when  the  steam  was  expanded  6  times  in  an 
individual  cylinder,  and  15  per  cent,  when  it  was  only  expanded  2J 
times.  Some  experiments  have  been  made  from  time  to  time  on 
the  economy  of  reheating  the  steam  between  the  cylinders,  and  this 
also  appears  to  be  considerable  under  certain  conditions.  The  steam 
jacket  operates  by  maintaining  the  temperature  of  the  cylinder  more 
nearly  uniform  throughout  the  stroke,  than  it  can  be  when  the 
cylinder  walls  are  subjected  only  to  the  varying  temperature  of  the 
steam  on  the  inside,  and  the  atmospheric  temperature  outside.  In 
the  case  of  reheating  the  steam  in  the  receivers  between  the 
cylinders,  the  object  to  be  attained  is  the  conversion  of  any  water 
that  may  be  formed  by  condensation,  into  steam  before  the  steam 
passes  on  to  the  next  cylinder,  and  with  it  the  reduction  of  condensa- 
tion in  that  cylinder.  With  many  engineers,  however,  steam 
jacketing  has  not  found  favour.  As  in  so  many  other  cases,  very 
much  depends  upon  how  the  steam  jacket  is  fitted  and  how  it  is 
worked.  In  the  case  of  some  compound  engines  which  came  under 
the  writer's  notice,  very  large  jackets  were  fitted,  and  the  engineer 
complained  that  while  the  warming  effect  on  the  cylinders  was 
comparatively  small,  the  waste  of  steam  by  condensation  in  the 
jacket  was  large.  In  other  cases  the  steam  jackets  have  been  too 
small,  and  the  warming  effect  on  the  cylinder  walls  has  not  been 
worth  the  loss  of  steam  pressure  due  to  the  passage  of  the  steam 
through  the  jacket.  Whether  the  steam  jacket  is  economical  or  not 
depends  upon  the  thermal  insulation  of  the  steam  cylinder  outside 
the  jacket.  And  this  brings  us  to  another  matter  which  is  too  often 


PLATE  4A. — Belliss'  Triple  Expansion  Engine,  directly  connected  to 
Three  Phase  Generator. 


PLATE  4B. — Parson's  Steam  Turbine,  as  made  by  Messrs.  Willans  &  Robinson,  with 
the  Turbine  Case  open  for  Inspection.     The  Governor  is  seen  on  the  Right. 

[To  face  p.  182. 


THE   GENERATION   OF   ELECTRICITY  129 

neglected  by  steam  engineers — the  thermal  insulation  of  the 
cylinders,  valves,  etc.  Every  engineer  is  familiar  with  the  warm 
atmosphere  that  nearly  always  pervades  a  steam  engine-house,  in 
marked  contrast  to  that  pervading  a  machine  house  of  any  kind 
where  the  driving  is  by  electric  motors.  The  engine  cylinders  and 
the  steam  pipes  are  sometimes  allowed  to  radiate  vigorously  into  the 
surrounding  atmosphere,  wasting  heat,  tending  to  greater  cylinder 
or  jacket  condensation,  and  creating  the  atmosphere  of  the  room  and 
the  draughts  that  are  often  so  trying.  Proper  insulation  of  the 
engine  cylinder,  steam  pipes,  valves,  etc.,  would  add  very  consider- 
ably to  the  economy  of  the  plant,  and  the  comfort  of  those  working 
in  it.  Steam  engineers  appear  to  be  always  alive  to  the  importance 
of  insulating  their  boilers,  where  the  boiler  shell  is  exposed  to  the 
atmosphere,  and  their  steam  pipes,  though  this  is  not  always  done  ; 
but  there  are  still  some  who  have  not  yet  appreciated  the  importance 
of  insulating  the  engines  themselves. 


Superheating 

By  superheating  is  understood  the  delivery  of  heat  to  the  steam 
after  it  has  left  the  steam  space  of  the  boiler.  In  the  steam  drum 
of  the  water-tube  boiler,  and  in  the  steam  space  in  Lancashire  boilers, 
the  steam  is  in  direct  contact  with  the  water,  and  is  continually 
receiving  additions  of  watery  vapour,  rising  from  the  water  with 
which  it  is  in  contact,  particularly  when  rapid  steaming  is  taking 
place.  In  all  superheating  apparatus  the  steam,  after  it  has  left  the 
steam  space  of  the  boiler,  is  made  to  pass  through  a  number  of  pipes, 
usually  of  smaller  diameter,  on  its  way  to  the  main  steam  pipe 
supplying  the  engines.  The  small  pipes  are  subjected  either  to  the 
heat  of  the  gases  from  the  boiler  furnace  itself,  or  to  heat  generated 
in  a  special  furnace  arranged  for  the  purpose.  In  several  of  the 
water-tube  boilers  the  superheater  consists  of  coils  of  pipe,  suspended 
in  the  space  inside  the  outer  brick  setting,  near  the  exit  of  the  gases 
to  the  chimney  or  economizer.  When  a  separate  furnace  is  employed, 
the  arrangement  is  very  similar  to  that  of  an  ordinary  boiler  furnace, 
but  in  place  of  the  usual  boiler  tubes,  the  superheating  tubes  are 
fixed  so  as  to  obtain  the  full  benefit  of  the  heat  from  the  gases 
produced  in  the  furnace.  The  primary  object  of  superheating  the 
steam  is  the  elimination  of  the  watery  vapour  that  has  been 
mentioned,  and  that  so  promptly  condenses  when  the  temperature 
is  lowered.  It  is  claimed  by  the  advocates  of  superheating  that  the 
steam  itself  receives  no  heat  until  the  whole  of  the  water  which  it 
carries  in  suspension  has  been  converted  into  steam,  and  the  idea 
has  arisen  among  some  steam  engineers  that  steam  which  has  been 

K 


1 3o 


ELECTRICITY   IN   MINING 


passed   through   a  superheater  must  necessarily  be   dry.      In   the 
writer's  opinion  this  is  not  strictly  correct.     The  specific  heat  of 


FIG.  66. — Tinker's  Superheater,  as  fitted  to  a  Lancashire  Boiler.  The 
Steam  to  be  heated  passes  through  the  Coils  of  Pipes  shown,  the 
Pipes  being  fixed  in  the  Path  of  the  Hot  Gases  from  the  Boiler  Flues 
to  the  Chimney. 

water  being  I/O,  while  that  of  steam  is  0*4,  any  water  that  is  present 
will  absorb  a  larger  quantity  of  heat  in  proportion  to  its  weight  than 


THE   GENERATION   OF  ELECTRICITY  131 

the  steam  which  surrounds  it ;  but  in  the  writer's  view  it  would  be 
contrary  to  all  the  laws  governing  the  transmission  of  heat  if  the 
steam  also  did  not  receive  a  certain  quantity  of  heat,  proportional 
to  the  difference  of  temperature  between  itself  and  the  pipes  in 
which  it  is  passing,  and  to  its  own  specific  heat.  It  is  evident 
that  if  a  certain  quantity  of  vapour  is  present  with  the  steam, 
sufficient  heat  must  be  delivered  to  each  pound  of  steam  and 
water  vapour  passing  through  the  superheater  to  convert  the  whole 
of  the  water  vapour  into  steam,  and  in  addition  to  this  an  allowance 
must  be  made  for  heat  delivered  to  the  steam  itself.  The  result 


FIG.  67. — Longitudinal  Section  of  Messrs.  Davey,  Paxman,  &  Co.'s  Separately 
Fired  Steam  Superheater.  The  Steam  passes  through  the  Coils  of  Pipe 
shown,  the  Hot  Gases  passing  up  and  all  round  them. 

of  superheating  the  steam  is  distinctly  satisfactory,  and  apparently 
within  certain  well-known  limits,  it  is  perfectly  safe  and  economical 
to  deliver  plenty  of  heat  to  the  steam  under  treatment.  It  ensures 
that  all  the  water  shall  be  converted  into  steam,  and  whatever 
heat  may  be  delivered  to  the  steam,  over  and  above  that  necessary 
for  the  conversion  of  the  water,  is  usefully  employed  in  raising  the 
temperature,  and  with  it  the  pressure  or,  per  contra,  the  volume 
of  the  steam  itself,  so  that  in  any  case  it  should  be  able  to  do  more 
work.  When  the  steam  that  is  being  superheated  is  confined, 
so  that  its  volume  cannot  increase,  temperature  and  pressure  will 


132 


ELECTRICITY  IN   MINING 


rise,  and,  providing  that  the  full  benefit  of  the  increased  pressure  can 
be  obtained  by  expansion,  economy  results.  When  the  steam  is 
not  confined,  say  when  it  is  being  superheated  on  its  way  to  the 
steam  cylinder,  expansion  will  take  place,  and  the  steam  engine 


h- 


FIG.  68. — Transverse  Section  and  Diagram  of  the  Front  of  Messrs.  Davey, 
Paxman,  &  Co.'s  Separately  Fired  Superheater. 

should  receive  the  benefit  of  the  larger  volume  of  steam  produced. 
As  with  all  improvements,  there  are  difficulties  in  the  way  of  the 
application  of  superheating  steam,  but  they  are  easily  overcome. 
Superheated  steam  is  at  high  temperatures,  from  450°  Fahr.  upwards, 
and  brass  and  gun-metal  will  not  stand  those  temperatures.  They 
disintegrate,  in  some  cases  practically  breaking  up  into  a  powdery 
mass.  In  addition,  only  certain  oils  will  stand  the  high  temperatures. 
The  difficulty  of  the  valves,  etc.,  has  been  overcome  by  the  use  of 
cast-iron  specially  prepared  for  the  purpose,  and  the  matter  of  the 
oil  has  been  overcome  by  the  use  of  special  mineral  oils.  According 
to  Professor  Siebel,  the  great  authority  on  refrigeration,  superheating 


THE   GENERATION   OF  ELECTRICITY 


133 


the  steam  increases  the  efficiency  of  the  engine  using  it  in  another 
way.  Superheated  steam,  the  professor  states,  has  only  one-fortieth 
the  conductive  ability  for  heat,  to  or  from  itself,  that  ordinary 
saturated  steam,  as  it  would  come  from  the  boiler,  has.  Hence  it 
would  give  off  much  less  of  its  heat  to  the  cylinder  walls,  even  when 


SUPERHEAT  DECREES  ran* 

pIGi  69. — Curves  showing  the  Advantages  of  different  Degrees  of  Superheat. 
Reproduced  by  Permission  from  Messrs.  Belliss  &  Morcom. 

they  are  at  a  much  lower  temperature.  Figs.  66,  67,  and  68  show 
two  forms  of  superheater,  one  heated  by  the  boiler  furnace  gases,  and 
one  by  a  separate  furnace,  and  Fig.  69  curves  showing  the  advantages 
of  superheating. 


ELECTRICITY   IN   MINING 


Condensers 

Another  source  of  economy  in  steam  generation  is  the  condenser. 
The  object  of  the  condenser  is  to  reduce  the  pressure  in  front  of  the 
piston  on  the  return  stroke.  Steam  enters  the  cylinder  behind  the 
piston  for  a  certain  portion  of  the  stroke.  Its  entry  is  then  stopped, 
and  it  continues  to  drive  the  piston  to  the  end  of  the  stroke  by  its 
own  expansion.  And  when  the  end  of  the  stroke  is  reached,  and  the 
piston  commences  its  next  stroke,  it  has  to  drive  the  steam  remaining 
in  the  cylinder  in  front  of  it,  out  through  the  exhaust  port,  and  into 


FIG.  70. — Edwards'  Triple  Air  Pump,  driven  by  Electric  Motor  as 
made  by  Messrs.  Isaac  Storey  &  Son. 

the  atmosphere,  unless  the  steam  is  got  rid  of  by  condensation  as 
soon  as,  or  very  quickly  after,  the  return  stroke  commences.  This 
means  that  the  piston  on  its  return  stroke  has  to  overcome  the 
pressure  remaining  in  the  steam,  plus  that  of  the  atmosphere,  and 
the  mean  effective  pressure  available  for  driving  the  piston  forward, 
is  the  mean  of  the  successive  pressures  behind  it,  less  the  mean  of 
the  successive  pressures  in  front  of  it.  If  the  back  pressure  of  the 
steam  can  be  extinguished,  it  is  equivalent  to  adding  a  certain 
number  of  pounds  pressure  to  that  of  the  steam  behind  the  piston ; 
and  if,  in  addition,  the  pressure  of  the  atmosphere  can  also  be 
partially  extinguished,  this  amount — 5,  10,  12,  or  13  Ibs.,  as  the  case 
may  be — is  also  added  to  the  effective  pressure  behind  the  piston. 


THE  GENERATION    OF  ELECTRICITY 


But  it  is  not  always  economical  to  condense.  It  is  only  economical 
when  the  saving  in  the  cost  of  generating  a  horse-power,  obtained  by 
condensing,  is  greater  than  the  cost  per  horse-power  of  condensing. 
In  all  condensers  water  is  employed  for  cooling,  and  water  is  often 
a  very  expensive  commodity.  In  addition,  the  circulating  water  has 
to  be  pumped,  and  the  power  to  drive  the  pump  must  come  originally 
from  the  furnace  of  the  boiler,  and  will  require  its  own  quota  of  coal. 
Further,  for  efficient  condensation  of  steam,  an  air  pump  is  necessary. 
The  steam  is  not  instan- 
taneously converted  into 
water  and  allowed  to  run 
away  harmlessly.  In  the 
jet  and  surface  condensers 
it  has  to  be  pumped  out 
of  the  condenser  in  which 
it  is  formed,  and  delivered 
to  the  hot  well,  or  to 
whatever  receptacle  it  may 
be  consigned  to;  and,  in 
addition,  the  air  which  is 
mixed  more  or  less  with 
the  steam,  and  which  re- 
mains mixed  with  the 
water  into  which  the  steam 
is  converted,  has  also  to  be 
pumped  out,  and  the  air 
pump  has  to  be  driven,  the 
power,  as  in  the  case  of  the 
circulating  pump,  coming 
from  the  boiler  furnace  in 
the  first  instance.  Fig.  70 
shows  a  motor-driven  triple  Fm  71._Section  of  Edwards'  Air  Pump,  showing 
air  pump,  and  Fig.  71,  a  the  Form  of  the  Piston, 

section  of  the  Edwards  air 

pump,  the  one  almost  universally  employed  for  condensers.  It  is  a 
very  instructive  sight,  in  the  matter  of  condensing  for  steam  purposes, 
to  visit  a  large  power  station  where  steam  turbines  are  employed. 
The  condensing  plant  occupies  a  much  larger  space  than  the  turbines, 
and  the  power  absorbed  by  it  is  decidedly  appreciable. 


Forms  of  Condenser 

There  are  three  principal  forms  of  condenser  employed  with  steam 
engines — the  surface  condenser,  the  jet  condenser,  and  the  ejector 
condenser.  The  surface  condenser,  again,  is  made  in  two  forms,  the 


ELECTRICITY   IN   MINING 


•as 


11 

II 


I! 
it 


is 


s 

§3 


THE   GENERATION    OF   ELECTRICITY 


137 


submerged  condenser  and  the  evaporative  condenser.    The  best  known 

forms  of  non-evaporative  surface  condensers  consist  of  boxes  of  pipes, 

the  water  circulating  through  the  pipes,  and  the  steam  passing  through 

the  box  in   the   space   left   vacant  by  them.     In   the    evaporative 

condenser,  of  which  the  Ledward  is  one  of  the 

best  known,  and  which  is  shown  in  Fig.  72, 

there  are  successive  coils  of  pipe,  corrugated 

on  the  outside,  standing   above   a  tank,  and 

having  above  them  a  perforated  pipe,  through 

which  the  water  is  distributed   to   the  upper 

sections  of  the  steam  pipe.     The  water  trickles 

down  over  the  corrugations  in  the  steam  pipe, 

and  is  collected  in  the  tank  at  the  bottom,  and 

made   to   do  duty  over   and  over  again,   the 

amount  lost  by  evaporation  being  made  good 

from  the  water-supply  service,  or  cooling  tower. 

The  air  pump  is  connected  to  one  end  of  the 

steam  pipe,  the  exhaust  steam  to  the  other. 

In  the  jet  condenser  the  steam  enters  the 
vessel  in  which  it  is  to  be  condensed,  and 
meets  a  jet  of  water  broken  up  into  a  fine  spray, 
the  two  mingling,  the  steam  being  condensed, 
and  the  whole  being  removed  by  a  pump, 
arranged  to  carry  off  the  circulating  water,  the 


FIG.  73.— Section  of  Worthington  Independent  Jet  Condenser. 

condensed  steam,  and  the  air.  Fig.  73  shows  a  Worthington  jet 
condenser.  In  the  ejector  condenser  the  exhaust  steam  is  made  to 
impinge  upon  a  stream  of  water  passing  through  a  vessel,  into  which 
the  steam  enters,  the  condensed  steam  being  carried  off  with  the  stream 
of  water.  Fig.  74  shows  a  Ledward  ejector  condenser.  The  quantity 


'38 


ELECTRICITY  IN   MINING 


of  cooling  water  required  for  each  form  of  condenser  will  vary  with  the 
temperature  of  the  water.     It  is  from  twenty  to  thirty-six  times  the 


FIG.  74.— Section  of  Ledward  Ejector  Condenser. 

weight  of  steam  for  surface  condensers.   For  jet  condensers  the  injec- 
tion water  allowed  is  from  twenty-seven  to  thirty  times  the  weight  of 


THE   GENERATION   OF  ELECTRICITY 


139 


steam  in  temperate  climates,  and  from  thirty  to  thirty-five  times  in 
the  tropics.  For  ejector  condensers  it  is  from  thirty-seven  to  forty  times 
the  weight  of  the  steam  to  be  condensed.  The  surface  condenser,  and 
principally  the  non-evaporative  form,  finds  most  favour  with  engineers, 
particularly  those  who  have  had  to  design  power  stations,  though  the 
evaporative  condenser  is  gradually  making  its  way.  There  are,  how- 
ever, a  good  many  jet  and  ejector  condensers  in  use,  one  important 
feature  about  them 
being  the  small  space 
they  occupy.  The 
surface  condenser, 
both  evaporative  and 
non  -  evaporative,  is 
subject  to  one  serious 
drawback,  the  in- 
crease of  the  thermal 
resistance  bet  ween  the 
cooling  water  and  the 
steam,  due  to  the 
deposit  of  grease  and 
other  substances  on 
the  surface  of  the 
pipes.  In  the  case 
of  the  evaporative 
condenser,  oxidation 
takes  place  on  the 
outer  surface  of  the 
pipes,  and  a  deposit 
of  grease  on  the  inner 
surface,  grease  being 
almost  invariably 
brought  over  from  the 
steam  cylinder  by  the 
steam,  in  the  form  of 
small  globules  of  oil, 
from  that  employed 
for  lubricating  the 


FIG.  75.— Section  of  Worthington  Central  Condenser. 


piston,  in  the  same  manner  as  it  carries  over  the  globules  of  water  in 
the  boiler.  The  deposit  of  grease  upon  the  outer  surface  of  the  pipes 
of  non-evaporative  condensers  is  sometimes  so  great  as  to  reduce  the 
vacuum  by  as  much  as  10  to  12  inches.  The  drawback  to  the  jet 
and  ejector  condensers  is,  that  they  are  not  so  economical  as  the 
surface  condensers  with  varying  load,  and  particularly  where  sudden 
heavy  loads  may  come  on  at  any  time.  It  is  necessary  in  both  forms 
that  the  quantity  of  water  passing  into  the  condenser  shall  be  sufficient 


140 


ELECTRICITY   IN    MINING 


to  condense  the  largest  quantity  of  steam  that  can  be  delivered 
to  the  condenser  at  any  time.  Hence,  where  the  load  varies  from 
very  small,  at  certain  times  of  the  day,  to  very  sudden  heavy  loads 
at  certain  other  times,  the  engineer  is  obliged  to  keep  such  a  large 
quantity  of  water  in  circulation  that  the  economy  is  very  often 
doubtful.  A  case  was  mentioned  to  the  writer  of  an  electricity 


FIG.  76.— Sectional  Diagram  of  Worthington  Central  Condensing  Station 
with  Pumps  and  Piping. 

generating  station  in  which  an  ejector  condenser  was  in  use.  The 
engines  employed  were  working  very  close  to  their  full  power,  and  it 
happened  on  one  occasion  that  one  of  them  broke  down,  but  by  dis- 
connecting the  condenser  and  stopping  its  pump,  the  remaining 
engines  were  able  to  deal  successfully  with  the  heaviest  load  the 
station  had.  Complaints  are  made  also  at  times  of  both  jet  and 


THE   GENERATION   OF  ELECTRICITY  141 

ejector  condensers  suddenly  losing  their  vacuum.  It  should,  perhaps, 
be  mentioned  that  central  condensing  stations  are  being  erected  in 
different  parts  of  the  kingdom,  to  deal  with  the  steam  from  groups  of 
plants.  They  are  usually  on  the  jet,  or  ejector  principle.  The  whole 
of  the  steam  from  the  different  plants  is  led  to  the  condensing  station, 
and  there  dealt  with,  usually  in  one  or  more  large  condensers.  The 
economy  of  this  arrangement  arises  from  the  variation,  or  what  the 
present  writer  has  termed  the  user  factor.  When  a  number  of  engines 
are  at  work,  it  is  not  often  that  they  are  all  taking  steam  at  the  same 
rate  at  the  same  instant,  and  hence,  if  the  steam  from  their  exhausts 
can  all  be  led  to  one  condensing  plant,  considerable  economy  in 
cooling  water  can  be  obtained.  Fig.  75  shows  a  section  of  a  Worth- 
ington  central  condenser,  and  Fig.  76,  a  Worthington  central 
condensing  station. 


Steam  Turbines 

There  are  practically  four  forms  of  steam  turbine  at  present  on 
the  market,  and  there  are  two  main  lines  upon  which  steam  turbines 
are  constructed,  two  of  those  on  the  market,  the  Parsons  and  De  Laval, 
illustrate  the  two  principles,  they  being  constructed  entirely  in  accord- 
ance with  those  principles,  while  the  others  are  more  or  less  modifica- 
tions, in  which  both  principles  are  made  use  of  to  a  certain  extent. 
In  the  Parsons  turbine  there  is  a  shaft  carrying  upon  it  a  number  of 
circles  made  up  of  fan  blades,  the  circles  being  of  different  diameters, 
as  they  are  farther  and  farther  from  the  steam  entry  port.  The  fan 
blades  on  the  shaft  run  between  fan  blades  fixed  on  the  inner  side  of 
a  cylindrical  containing  vessel,  the  cylinder  becoming  larger  as  it 
recedes  from  the  entry  port.  The  steam  enters  at  one  end  of  the 
cylinder,  and  it  passes  through  the  circles  of  fan  blades  and  the  shaft 
in  succession,  and  in  doing  so  it  exerts  a  certain  pressure  upon  each 
individual  fan  blade,  the  pressure  being  communicated  to  the  shaft, 
the  result  being  a  turning  movement.  The  pressure  of  the  steam  is 
resolved  into  two  forces  at  right  angles  to  each  other,  and  it  is 
the  force  in  the  direction  of  rotation  of  the  shaft  which  causes 
it  to  move.  The  steam,  as  it  passes  through  each  circle  of  fan 
blades,  loses  a  certain  portion  of  its  pressure,  and  it  is  for  this  reason 
that  the  successive  batches  of  circles  of  fan  blades  are  larger,  as 
the  distance  from  the  steam  entry  port  increases.  The  action  is 
exactly  the  same  as  that  of  the  steam  in  a  compound  engine,  and  the 
fan  blades  are  made  larger,  so  that  the  turning  moment  exerted  by 
the  steam  upon  each  successive  length  of  the  shaft  shall  be  as  nearly 
the  same  as  possible,  the  increased  diameter  of  the  circles  of  fan  blades 
making  up  for  the  decreased  pressures.  In  the  Parsons  turbine  the 


i42  ELECTRICITY  IN   MINING 

steam  is  admitted  to  the  turbine  cylinder  in  gusts,  and  not  in  a 
continuous  stream.  The  duration  of  each  gust  is  controlled  by  the 
governor,  which  again  is  controlled,  either  electrically  by  means  of  a 
solenoid,  or  by  an  ordinary  governor  of  the  centrifugal  type.  The 
core  of  the  solenoid,  when  an  electrical  governor  is  employed,  is  hung 
from  the  end  of  a  long  lever,  the  short  arm  of  which  controls  the 
valve  of  the  steam  relay.  At  regular  intervals,  which, may  be  ad- 
justed, the  steam  relay  admits  a  certain  quantity  of  steam  to  the 
turbine,  and  the  valve  is  then  immediately  closed,  the  quantity  of 
steam  admitted  being  controlled  by  the  time  the  valve  is  opened, 
very  much  as  with  an  expansion  governor  controlling  a  slide  valve. 
Where  the  ordinary  centrifugal  governor  is  employed,  practically  the 
same  arrangement  holds,  but  it  is  not  controlled  by  variations  in  the 
electric  circuit  as  with  the  electrical  governor.  It  is  controlled  simply 
by  variations  in  the  load,  increasing  or  decreasing  the  speed  of  the 
turbine,  and  spreading  the  arms  of  the  governor,  or  the  reverse. 
When  the  steam  turbine  is  employed  to  drive  an  alternating  current 
generator,  an  additional  solenoid  is  added  to  the  governor,  connected  in 
series  with  the  work,  the  solenoid  of  the  continuous  current  governor 
being  connected  as  a  shunt.  The  Parsons  turbine  is  fixed  on  the  same 
bed-plate  with  the  generator  it  is  to  drive,  its  shaft  being  coupled 
directly  to  the  shaft  of  the  dynamo  by  a  steel  sleeve.  The  dynamo  is, 
of  course,  constructed  to  run  at  the  highest  speeds  at  which  the  turbine 
runs,  and  this  was  for  some  time  a  serious  difficulty  in  the  matter  of 
upkeep,  the  question  of  the  brush  contacts  being  a  somewhat  serious 
one  at  high  speeds ;  this,  however,  has  been  completely  overcome. 
One  of  the  troubles  in  connection  with  a  turbine  of  the  pressure  type, 
such  as  Parsons  turbine,  is  the  matter  of  end  thrust  of  the  shaft. 
Until  recently,  in  the  Parsons  turbine,  end  thrust  was  provided  for 
by  the  provision  of  grooved  pistons  or  dummies,  on  the  end  of  the 
turbine  shaft,  which  fitted  into  grooves  in  the  turbine  case,  and 
which,  it  was  claimed,  provide  a  steam-tight  joint.  Now  it  is  balanced 
by  pistons  behind  the  entry  port. 


The  Willans-Parsons  Steam  Turbine 

Messrs.  Willans  &  Eobinson  manufacture  a  modification  of  the 
Parsons  turbine,  one  feature  of  which  is  the  construction  and  general 
arrangement  of  the  moving  and  guide  blades.  The  individual  blades 
are  stamped  out  with  a  dove- tail  section  or  "  tang  "  at  the  end  which 
is  to  be  fixed  either  to  the  axle  or  to  the  casing,  and  at  the  other  end 
is  a  short  tongue  provided  for  riveting  into  a  shrouding.  The  blades, 
instead  of  being  held  simply  by  one  end  to  the  axle  or  the  turbine 
case,  are  held  between  two  rings,  one  called  the  foundation  ring  and 


THE   GENERATION   OF   ELECTRICITY  143 

the  other  the  shrouding  ring.  The  foundation  ring  is  secured  to  the 
axle  or  the  turbine  case,  and  the  dove-tail  section  part  of  each  blade 
is  fixed  in  the  foundation  ring,  and  held  there  while  the  outer  ends 
of  the  blades  are  secured  by  the  short  tongues  mentioned,  being  passed 
through  holes  in  the  shrouding  ring.  Successive  rings  built  up 
in  this  way  are  held  on  the  axle  and  on  the  casing,  the  rings,  as 
before,  increasing  in  section  as  the  pressure  of  the  steam  decreases. 
The  rings  containing  the  moving  blades  are  thus  practically  held 
between  the  guide  blades,  the  clearance  space  between  the  two  being 
very  small,  and  the  necessary  angle  for  the  moving  and  guide  blades 
being  arranged  in  fixing  them  between  the  foundation  and  shrouding 
rings.  The  moving  blades  run  practically  as  discs  might  do  between 
other  discs.  The  angles  of  the  moving  and  guide  blades  are  so 
arranged  as  to  give  the  maximum  effort  from  the  steam  passing 
through  the  successive  rings  of  blades  in  the  direction  of  rotation. 
Messrs.  Willans  &  Eobinson  prefer  to  govern  the  admission  of  steam 
by  a  very  powerful  centrifugal  governor  of  the  ordinary  type,  having 
ball-bearings  on  all  its  working  parts,  the  governor  acting  upon  the 
throttle  valve,  and  being  driven  by  worm  gearing  on  an  extension  of 
the  main  turbine  shaft.  The  turbine  case  is  made  in  two  halves 
longitudinally,  the  dividing  plane  being  horizontal,  so  that  the 
upper  half  of  the  case  can  be  thrown  back  by  removing  the  bolts 
holding  it  in  position,  and  the  blades  examined  at  any  moment,  as 
shown  in  Plate  4fi. 

Messrs.  Willans  &  Robinson's  turbines  are  made  for  outputs  of 
from  250  to  7500  kilowatts,  speeds  ranging  from  600  revolutions  in 
the  larger  turbines  to  3000  in  the  smaller. 


The  De  Laval  Turbine 

In  the  De  Laval  turbine  the  pressure  of  the  steam  is  expanded 
down  to,  or  nearly  to,  atmospheric  pressure  before  it  is  made  to 
operate  the  turbine  wheel.  The  turbine  itself  consists  of  a  disc,  upon 
the  periphery  of  which  are  fixed  small  buckets,  very  similar  in  form, 
though  much  smaller,  than  those  of  some  forms  of  water  wheel.  The 
turbine  disc  with  its  buckets  is  enclosed  inside  a  case,  and  the  steam 
is  delivered  to  the  buckets  from  nozzles  fixed  on  one  side  of  the 
turbine  disc.  The  nozzles  are  so  formed  that  the  molecules  of  the 
steam  as  nearly  as  possible  take  a  straight  path  from  the  nozzle  into 
the  turbine  bucket,  and  the  work  the  turbine  is  doing  may  be  con- 
trolled by  using  a  greater  or  less  number  of  the  nozzles.  The 
operation  of  the  turbine  is  very  similar  to  that  of  some  forms  of  water 
wheel.  The  buckets  are  filled  with  steam  which  is  issuing  from  the 
nozzle  at  considerable  velocity,  and  the  weight  and  velocity  of  the 


144  ELECTRICITY  IN   MINING 

steam  push  the  buckets  away  from  the  nozzle,  the  steam  escaping  on 
the  other  side  of  the  disc  to  the  condenser.  The  De  Laval  turbine  is 
always  geared  down  by  means  of  spur  gearing  contained  in  a  gear 
chamber,  fixed  on  the  same  bed  plate  as  the  turbine,  the  power  being 
taken  from  the  axle  of  the  last  wheel  of  the  gearing,  the  second  motion 
shaft.  The  turbine  nozzles  have  valves  attached  to  them  by  means 
of  which  the  quantity  of  steam  passing  through  them  can  be  controlled, 
so  that  variation  of  load  is  provided  for  by  changing  the  number  of 
nozzles  at  work,  and  by  throttling  the  steam  passing  through  the 
nozzles,  or  the  reverse.  The  governor  is  of  the  centrifugal  type,  with 
special  arrangements  for  sensitive  government,  and  assisted  by  a 
vacuum  valve  when  the  turbine  is  run  condensing. 


The  Curtis  Turbine 

The  Curtis  turbine  may  be  taken  as  a  modification  of  the  De  Laval 
turbine,  but  with  special  arrangements  allowing  the  steam  to  work 
expansively,  as  in  the  Parsons  turbine.  The  turbine  consists  of  a 
number  of  moving  discs  fixed  horizontally  upon  a  vertical  shaft,  moving 
above  a  certain  number  of  stationary  discs,  each  disc  having  a  certain 
number  of  buckets,  similar  to  those  on  the  De  Laval  turbine,  on  its 
periphery,  the  steam  passing  in  succession  through  the  moving  and 
stationary  discs.  The  successive  discs  increase  in  size  from  the  top 
where  the  steam  enters  to  the  bottom  where  it  leaves,  the  whole 
apparatus  being  fixed  vertically.  The  discs  are  arranged  in  sets,  and 
between  successive  sets  there  is  a  diaphragm  which  practically  makes 
each  section  a  separate  turbine  with  its  own  steam  chest.  The  Curtis 
turbine  is  employed  principally  for  driving  electric  generators,  at 
which  it  has  achieved  a  considerable  amount  of  success,  the  generator 
being  fixed  above  the  upper  disc  of  the  turbine,  and  arranged  for  its 
armature  or  moving  member  to  rotate  in  a  horizontal  plane  upon  a 
vertical  axis,  the  shafts  of  the  generator  of  the  turbine  being 
mechanically  connected,  as  in  other  apparatus  that  have  been 
described.  The  Curtis  turbine  for  any  given  size  occupies  a  much 
smaller  floor  space  than  any  other  form  of  steam  motor,  but  it  is 
claimed  by  makers  of  other  forms  of  turbine  and  of  reciprocating 
engines  that  very  little  advantage  is  gained,  since  the  foundations 
upon  which  the  turbine  rests  must  be  stronger  in  proportion. 
Further,  a  special  floating  bearing  must  be  provided  for  the  lower  end 
of  the  vertical  spindle  of  the  turbine,  and  the  lubrication  of  this 
bearing  requires  very  careful  attention.  In  practice  the  lower  end  of 
the  spindle  is  supported  by  what  is  practically  hydraulic  pressure. 
The  Curtis  turbine  is  usually  governed  electrically. 


THE   GENERATION   OF  ELECTRICITY  145 


The  Westinghouse  Turbine 

The  Westinghouse  turbine  is  another  modification  in  which 
some  of  the  features  of  the  Parsons  and  some  of  those  of  the  Curtis 
are  included.  The  turbine  case  is  cylindrical,  and  stands  horizon- 
tally, very  much  as  the  Parsons  turbine.  Steam  is  led  into  the 
turbine  in  the  centre,  and  there  are  a  number  of  circles  of  fan  blades 
fixed  upon  the  revolving  shaft,  the  circles  becoming  larger  in  section 
as  the  steam  entry  port  is  left  behind,  but  the  last  portion  of  the  work 
of  the  steam  is  performed  by  discs  very  similar  to  those  of  the  Curtis 
turbine,  except  that  they  are  fixed  vertically  upon  horizontal  axes, 
with  the  buckets  fixed  on  the  edge  of  their  peripheries.  The 
remainder  of  the  arrangement  is  very  similar  to  that  described  for 
other  turbines. 


Turbines  using  Exhaust  Steam 

Professor  Kateau  has  made  this  subject  particularly  his  own,  and 
the  arrangement  he  has  devised  is  a  turbine  which  is  a  modified  form  of 
those  that  have  been  described,  and  is  employed  to  drive  any  apparatus, 
such  as  a  dynamo,  a  fan,  or  anything  about  a  mine,  the  turbine  using 
the  exhaust  steam  from  the  winding  and  other  engines.  It  has  been 
explained  in  previous  pages  that  a  large  amount  of  energy  remains  in 
the  steam  which  exhausts  to  the  atmosphere,  and  it  is  this  energy 
that  is  utilized  in  the  Eateau  turbine.  There  are  certain  difficulties, 
however,  as  mining  engineers  will  understand,  in  the  application  of 
exhaust  steam  owing  to  the  intermittent  working  of  the  engines  about 
the  mine,  and  therefore  varying  the  supply  of  exhaust  steam.  To 
meet  this,  Professor  Eateau  has  designed  a  thermal  storage  apparatus, 
consisting  of  a  boiler  shell  loaded  with  old  iron  rails,  sleepers,  etc. 
When  the  exhaust  steam  is  more  than  is  required  for  driving  the 
turbine,  the  surplus  is  carried  into  the  thermal  store,  and  is  used  up 
in  heating  the  mass  of  iron.  When  the  supply  of  exhaust  steam 
runs  short,  the  pressure  in  the  thermal  store  being  reduced,  steam  is 
formed  there,  and  is  supplied  to  the  turbine,  making  up  what  is 
wanting  from  the  engine.  It  is  a  necessary  condition  of  using  this 
arrangement,  that  the  turbine  shall  be  employed  to  perform  work  that 
can  be  dealt  with  by  a  little  less  than  the  average  quantity  of 
exhaust  steam  throughout  the  day.  In  addition  to  this,  Professor 
Eateau,  in  the  installations  that  he  has  laid  down,  and  in  those  that 
have  been  established  in  this  country,  provides  for  an  automatic 
supply  of  steam  from  the  boiler  service,  if  the  supply  from  the 
exhaust  and  from  the  thermal  store  falls  below  a  certain  pressure. 

L 


146  ELECTRICITY   IN   MINING 


Steam  Turbines  and  Condensing 

One  of  the  most  important  features  in  connection  with  steam 
turbines  is  the  fact  that,  while  they  run  very  economically  when  the 
steam  exhausts  into  the  condenser,  if  condensation  is  not  practicable 
the  economy  is  very  considerably  reduced,  the  steam  consumption 
going  up  very  considerably.  The  economy  of  the  steam  turbine 
also  increases  with  the  vacuum  maintained  in  the  condenser,  and 
it  is  claimed  by  makers  of  reciprocating  engines  that  the  cost  of 
creating  the  vacuum  sometimes  neutralizes  the  economy  of  steam 
consumption.  It  is  a  very  striking  experience  to  visit  a  large 
electricity  generating  station,  such  as  that  of  the  Metropolitan 
District  Kailway  at  Lot's  Koad,  Chelsea,  where  steam  turbines  are 
employed.  The  steam  turbines  there  are  capable  of  generating 
5500  K.W.  each,  or,  say,  7000  H.P.,  and  the  small  space  occupied 
by  the  turbine  and  the  alternator  it  is  driving  is  very  marked,  but 
on  going  below  the  floor,  where  the  turbo-generators  are  fixed,  one 
finds  a  very  large  quantity  of  apparatus  on  the  floor  below,  which 
are  necessary  for  providing  the  condenser  vacuum  required  if  the 
turbines  are  to  work  economically,  the  space  occupied  by  the  con- 
densing plant  for  each  turbo  being  many  times  that  occupied  by  the 
turbo  itself,  and  the  power  required  for  each  condensing  plant  being 
very  appreciable.  It  is  claimed  by  Mr.  Parsons,  however,  who  has 
designed  special  apparatus  for  obtaining  high  vacua,  that  the  total 
cost  of  condensing  does  not  exceed  1 J  per  cent,  of  the  output,  while 
it  creates  an  economy  of  4  to  5  per  cent,  in  the  coal  bill. 


Cooling  Towers  and  Ponds 

The  crux  of  the  condenser  problem  is  nearly  always  the  cost  of 
the  cooling  water.  Where  the  town  service  is  the  only  water  avail- 
able, the  cost  is  nearly  always  prohibitive,  and  it  is  found  that  in 
large  towns,  unless  there  are  independent  sources  of  supply,  such  as 
wells  or  rivers,  condensing  is  not  employed.  In  London,  for  instance, 
the  exhaust  steam  is  very  often  delivered  into  the  chimney.  Where 
a  source  of  water,  such  as  a  stream,  is  available,  the  question  turns 
upon  the  cost  of  pumping.  An  instance  that  has  come  under  the 
writer's  notice  is  of  interest.  In  one  of  the  electricity  generating 
stations  at  Newcastle-on-Tyne,  water  is  pumped  from  the  river  for 
the  condensers,  and  it  is  allowed,  after  passing  through  the  con- 
densers, to  run  down  to  the  river  again.  The  generating  station  is 
about  90  feet  above  the  river  and  the  pumping  plant  is  threefold. 
There  is  a  centrifugal  pump  driven  by  an  electric  motor  taking 


THE   GENERATION   OF  ELECTRICITY  147 

current  from  the  generating  station,  and  on  the  same  axle  a  water 
turbine.  The  centrifugal  pump  delivers  the  water  at  the  generating 
station,  and  the  return  water  from  the  condenser  drives  the  water 
turbine,  so  that  the  electric  motor  has  only  to  make  up  the  difference 
between  the  net  power  delivered  by  the  turbine  to  the  common  axle 
and  the  power  required  by  the  centrifugal  pump. 

There  are  many  cases,  however,  where  water  is  scarce,  and  the 
cost  of  pumping  any  water  available  is  high;  and  where,  if  con- 
densing is  to  be  employed,  the  water  must  be  used  over  and  over 
again,  and  for  that  purpose  must  be  cooled  after  passing  through  the 
condenser.  Perhaps  the  simplest  and  most  economical  arrangement 
of  this  kind,  where  it  can  be  employed,  is  that  which  is  so  common 
in  connection  with  Lancashire  cotton  mills,  and  which  can  be  so 
easily  arranged  at  a  mine.  There  is  nearly  always  a  pond  holding 
a  large  quantity  of  water,  and  with  a  large  surface,  close  to  the 
engine-house.  The  cooling  water  is  pumped  from  the  pond  to  the 
condenser,  and  is  allowed  to  return  from  the  condenser  to  the  pond. 
Evaporation  is  constantly  taking  place  from  the  surface  of  the  pond, 
more  particularly  in  hot  weather,  and  this,  combined  with  the  large 
mass  of  water  employed,  is  sufficient  to  maintain  the  cooling  water  at 
a  sufficiently  low  temperature.  It  is  a  simple  calculation  to  find 
how  much  water,  and  what  size  pond,  with  what  extent  of  surface, 
must  be  provided.  Lancashire  cotton  mill  steam  plants,  it  is  well 
known,  are  perhaps  the  most  economical  power  generators  in  the 
world.  Another  method  is  by  the  aid  of  a  cooling  tower. 

The  cooling  tower  is  based  on  exactly  the  same  principles  as  the 
evaporative  condenser.  When  water  evaporates,  whether  it  is  con- 
verted into  steam,  or  into  vapour  that  is  held  in  the  atmosphere 
under  the  various  forms  we  are  familiar  with,  and  that  becomes 
visible  in  the  form  of  fog  and  of  cloud,  a  certain  definite  quantity  of 
heat  is  required  to  enable  each  pound  or  gallon  of  water  to  assume 
the  form  of  vapour.  The  actual  quantity  of  heat  required  to  form 
vapour,  as  distinguished  from  the  quantity  of  heat  required  to  form 
steam,  has  not,  the  author  believes,  been  accurately  determined,  but  it 
will  be  safe  to  assume  that  it  is  not  far  removed  from  the  quantity 
required  to  form  steam.  For  the  purposes  of  calculation,  and  as  a 
safe  guide,  the  author  is  accustomed  to  take  900  B.Th.  Units  per 
pound  of  water  evaporated  at  temperatures  lower  than  boiling-point. 
Secondly,  the  atmosphere  has  the  property  of  absorbing  a  certain 
quantity  of  moisture,  the  quantity  varying  with  the  temperature. 
The  quantity  does  not  increase  simply  in  the  same  ratio  as  the 
temperature ;  that  is  to  say,  at  40°  the  capacity  of  the  atmosphere 
for  absorbing  is  not  twice  that  at  20° ;  it  is  more  than  twice,  and  at 
80°  it  is  very  much  more  than  twice  that  at  40°.  The  rate  of  increase 
follows  a  hyperbolic  curve,  the  latter  portion  of  the  curve  being  very 


i48  ELECTRICITY   IN   MINING 

steep  indeed,  almost  a  vertical  line,  while  the  early  portion  is  almost 
a  horizontal  line,  the  consequence  being  that  the  rate  of  increase  of 
the  capacity  for  moisture  is  very  rapid  when   comparatively  high 
temperatures  are  reached.     This  means,  of  course,  that  warm   air, 
having   a  high  capacity  for  water  vapour,  has  also  considerable 
evaporative  effect,  and  therefore  a  considerable  cooling  effect.    The 
cooling  of  the  water  in  a  cooling  tower  is  effected  by  the  evaporation 
of  a  small  portion  of  the  water,  the  heat  required  in  this  case  being 
taken,  to  a  large  extent,  from  the  water  itself,  and  this  performing 
the   operation  of  cooling.     But   this   is   not  the  whole   story.     In 
addition  to  the  above,  every  cubic  foot  or  cubic  yard  of  air  has  its 
capacity  for  absorbing  moisture  at  each  temperature,  and  therefore, 
if  air  is  made  to  pass  over  the  surface  of  the  water  that  is  to  be 
cooled,  and,  in  particular,  if  the  water  is   broken   up,  as  will  be 
explained,  into  very  fine  particles,  so  that  the  air  can  reach  every 
particle,  or  a  large  number  of  them,  the  cooling  effect  will  then 
depend  upon  the  quantity  of  air  passing  through  the  cooling  tower, 
and  the  capacity  of  each  cubic  foot  of  air  for  absorbing  vapour  at  its 
then  temperature,  and  under  the  conditions  ruling.     But  there  is  yet 
another  factor  in  the  problem.     If  the  air  is  already  fully  saturated, 
if  it  has  already  absorbed  all  the  moisture  it  is  capable  of  at  that 
temperature,  it   cannot  absorb  any  more,  and  it  will  not  only  not 
produce  any  evaporative  effect,  and  therefore  no  cooling  effect,  but  it 
is  more  than  probable  that  the  air  itself  may  be  slightly  cooled  by 
coming  into  contact  with  bodies  that  will  absorb  some  of  its  heat, 
and  then  its  capacity  for  holding  moisture  being  lowered,  it  will 
deposit  vapour  upon  any  substance  that  is  at  hand,  in  this  case  upon 
the  particles  of  water,  and  in  place  of  cooling  the  water,  the  latent 
heat  of  the  vapour  deposited  from  the  air  will  be  delivered  to  the 
water,  and  will  raise  its  temperature  instead   of  lowering  it.     The 
question  whether  vapour  shall  be  deposited  in  the  form  of  water  from 
the  air,  or  whether  the  air  shall  absorb  water  in  the  form  of  vapour 
from  the  water  in   the  cooling  tower,  depends   upon   the  relative 
tensions  of  the  vapour  in  the  air,  and  the  vapour  that  is  issuing  from 
the  water.     Evaporation  takes  place  from  the  surfaces  of  liquids  at 
all  temperatures,  unless  it  is  prevented  by  pressure  upon  the  surface 
of  the  liquid,  the  pressure  in  question  being  exerted  by  the  vapour 
in  the  atmosphere,  or  gas,  impinging  upon  the  liquid.     When  vapour 
is  issuing  from  the  surface  of  a  liquid,  it  has  a  certain  tension,  and 
the  vapour  which  is   present  in   the  atmosphere  also  has  its  own 
tension.     When  the  tension  of  the  two  are  equal,  no  evaporation 
takes  place,  and  no  deposit  of  vapour  from  the  atmosphere.    When 
the  tension  of  the  vapour  emanating  from  the  liquid  is  greater  than 
that  of  the  vapour  already  present  in  the  atmosphere,  evaporation 
takes  place,  and  when  the  reverse  of  these  conditions  rules,  deposit 


THE   GENERATION   OF   ELECTRICITY  149 

takes  place.  The  tension  of  the  vapour  in  the  atmosphere  varies 
with  the  temperature,  and  with  the  quantity  of  vapour  already 
present. 

Forms  of  Cooling  Towers 

Cooling  towers  are  of  various  forms.  In  some  of  them  natural 
draught  is  made  use  of;  that  is  to  say,  the  tower  is  built  in  the 
form  of  a  chimney,  and  the  air  passes  through  the  chimney  for  the 
same  reason  that  it  passes  up  the  chimney  of  a  fireplace  or  a  furnace. 
In  other  forms,  fans  are  employed  to  force  air  up  the  tower,  the  fans 
being  driven  by  any  convenient  source  of  power,  an  electric  motor 
being  a  favourite  one,  though  a  small  steam  or  gas  engine  will  answer 
equally  as  well.  Where  the  cooling  tower  depends  upon  natural 
draught,  the  chimney,  which  may  be  of  wood,  iron,  brick,  or  any 
substance  that  is  preferred,  is  built  of  a  very  much  greater  height 
than  is  necessary  where  fans  are  employed,  and  for  the  same  reason 
as  with  forced  draught.  The  apparatus,  in  fact,  consists  of  two 
distinct  parts — the  cooling  portion  proper,  and  the  chimney  which  is 
to  create  the  draught.  In  all  forms  of  cooling  towers  the  water  is 
divided  up  by  various  devices,  and  is  made  to  trickle  down,  or  to 
fall  down  in  a  finely  divided  spray  from  the  top  of  the  cooling  tower, 
and  is  collected  in  a  tank  or  pond  at  the  bottom.  The  devices  for 
breaking  up  the  water  consist  principally  of  wooden  troughs,  some- 
times set  on  edge,  of  wooden  slats  set  on  edge,  of  mats  of  various  forms 
galvanized  iron  being  a  favourite  one,  also  hung  on  edge.  The  Worth- 
ington  Co.  use  glazed  pipes  standing  vertically  in  successive  rows,  as 
shown  in  Fig.  77.  The  water  is  either  carried  by  gravity,  where  that  is 
possible,  or  pumped  to  the  top  of  the  cooling  tower,  and  is  there 
delivered  to  the  upper  portion  of  the  arrangements  for  breaking  it  up, 
the  whole  of  the  tower  being  filled  with  the  slats  or  mats,  or  whatever 
may  be  employed,  and  the  water  dropping  from  one  to  the  other  in  its 
descent.  Whatever  the  form  of  cooling  tower,  the  rule  which  holds 
in  all  cases  of  this  kind  applies,  the  air  enters  at  the  bottom  of  the 
tower,  and  meets  the  water  as  it  descends,  the  coolest  air  meeting  the 
coolest  water  at  the  bottom,  and  the  air  that  is  charged  with  the 
largest  quantity  of  vapour  meeting  the  warmest  water  at  the  top. 
In  the  chimney  form  of  cooling  tower,  the  chimney  is  built  immedi- 
ately on  top  of  the  cooling  arrangements,  whatever  they  may  be,  and 
the  latter  are  sometimes  spread  out  so  as  to  occupy  a  large  area,  the 
chimney  rising  from  the  centre,  the  vapour,  as  usual,  issuing  from 
the  top  of  the  chimney  with  the  air.  There  is  also  another  type  of 
natural-draught  cooling  tower,  in  which  the  cooling  arrangements, 
the  laths,  etc.,  are  stretched  out  over  a  large  area,  and  in  which  the 
air  draught  is  obtained  from  the  force  of  the  wind  only,  there  being 


150 


ELECTRICITY  IN   MINING 


no  chimney.     In  this  form  of  cooling  tower,  louvre  boards  are  fitted 
at  each  end  of  the  tower,  and,  if  necessary  at  the  sides,  the  boards 

being  arranged  to 
open  or  close  by 
means  of  levers,  in 
a  similar  manner  to 
Venetian  blinds. 
When  there  is  very 
little  air  about,  the 
louvres  are  thrown 
wide  open,  and  they 
are  closed  more  or 
less,  according  to  the 
force  of  the  wind. 
The  cooling  towers 
in  which  fans  are 
employed  are,  as  ex- 
plained above,  shorter 
than  those  in  which 
chimney  draught  is 
made  use  of.  They 
have,  however,  a  short 
;<-«*  HOT  WATER,  chimney  containing  a 
water  baffle  above  the 
cooling  arrangements, 
the  object  being  to 
catch  any  water  that 
may  be  carried  up- 
wards by  the  mechani- 
cal action  of  the  air 
upon  the  water,  and  so 
lost  by  being  carried 
away.  The  fan  cool- 
ing towers  are  made 
of  wood,  iron,  brick, 
concrete,  and  any  other 
convenient  substance. 
The  fan  is  placed  at 
the  bottom  of  the 
tower,  the  water  being 
led  to  the  top,  and 
the  arrangements  for 
breaking  the  water  up 


COLO  WATER. 
* 


SUCTION  TAN* 


FIG.  77. — Section  of  Worthington  Cooling  Tower. 
The  Earthenware  Pipes  used  for  breaking  up 
the  Water  are  shown,  also  the  Fan  and  other 
Arrangements. 


into  minute  globules   being  the   same   as  in  the  chimney  cooling 
tower.     The  size  of  the  fan  will  be  a  matter  of  calculation,  and 


THE   GENERATION   OF  ELECTRICITY  151 

will  depend  upon  the  quantity  of  air  that  must  be  driven  through 
the  tower,  this  again  depending  upon  the  temperature  and  the 
tension  of  the  vapour  in  the  air,  as  opposed  to  that  of  the  water 
to  be  cooled.  The  fan  must  be  of  sufficient  size,  and  driven  by 
sufficient  power  to  provide  a  sufficiently  powerful  current  of  air 
under  the  worst  conditions  that  can  rule ;  that  is  to  say,  with  the 
smallest  difference  of  tension  between  the  two  vapours.  Cooling 
towers  are  made  sometimes  circular,  with  a  single  fan,  some- 
times two  circular  towers  are  placed  side  by  side,  each  with  its  own 
fan,  and  sometimes  a  rectangular  section  is  adopted,  the  tower 
being  divided  into  two  or  more  sections,  each  having  its  own  fan. 
The  air  for  the  cooling  tower  may  be  taken  directly  from  the 
atmosphere,  or  it  may  be  cooled,  or  warmed,  or  dried  on  its  way  to 
the  cooling  tower.  It  is  evident  that  once  it  is  determined  to  handle 
the  air  mechanically — that  is,  to  perform  the  operations  of  cooling 
mechanically — the  handling  can  be  extended,  and  the  air  can  be 
taken  through  any  other  apparatus,  such  as  for  warming  and  drying, 
that  may  be  convenient ;  but  it  must  be  borne  in  mind  that  the  cost 
of  warming  and  cooling  or  drying,  must  be  taken  account  of  in  the 
balance  sheet. 

The  question  of  condensing,  as  previously  explained,  depends  almost 
entirely  upon  the  cost  of  the  water  employed,  and  everything  which 
tends  to  add  to  the  cost  of  the  water  adds  to  the  cost  of  condensing, 
and  reduces  the  economy  and  advantage.  Messrs.  Korting  Bros, 
and  others  have  developed  an  arrangement  in  connection  with 
cooling  ponds,  in  which  pipes  are  laid  in  rows  longitudinally  across 
the  pond,  supported  on  baulks  of  wood  or  blocks  of  stone,  or  in  any 
convenient  way,  and  nozzles  are  fixed  at  intervals  along  the  pipes, 
the  water  issuing  in  sprays  from  the  nozzles,  and  falling  into  the 
pond.  Messrs.  Korting  state  that  with  17  feet  head  of  water,  each 
nozzle  will  provide  a  cooling  space  of  60  square  feet,  and  that  100 
gallons  per  hour  per  nozzle  may  be  cooled  from  a  temperature  of 
110°  Fahr.  to  75°  Fahr.,  with  a  loss  of  only  3  per  cent,  of  the  water 
sprayed.  This  method,  it  will  be  seen,  is  open  to  the  objection  that 
the  water  is  blown  away  by  the  wind  when  it  is  strong,  unless  some 
provision  is  made  for  protecting  it. 

All  forms  of  cooling  apparatus  are  subject  to  the  same  fault  that 
is  present  in  the  evaporative  condenser,  viz.  the  deposit  of  some  of 
the  salts  contained  in  the  water  upon  the  appliances  arranged  for 
breaking  the  water  up,  and  the  filling  up  of  holes,  the  filling  up  of 
troughs,  and  the  wearing  of  slats  and  of  mats,  by  chemical  action, 
where  there  are  any  salts  present  in  the  water,  and  for  this  reason 
wooden  boards  taking  the  form  of  somewhat  deep  troughs  of  triangular 
section,  and  similar  arrangements  have  found  more  favour  in  the 
eyes  of  many  engineers,  than  the  apparatus  in  which  a  more  perfect 


152  ELECTRICITY   IN   MINING 

breaking-up  of  the  water  into  spray  is  arrived  at,  because  these  forms 
of  apparatus  are  less  liable  to  get  out  of  order,  and  the  results  are 
less  liable  to  change  than  those  in  which  the  holes,  etc.,  are  liable 
to  be  partially  or  wholly  filled  up.  The  problem  involved  in  hand- 
ling cooling  water,  though  the  calculations  are  very  simple,  is  itself 
by  no  means  so  simple  as  it  looks  at  first  sight.  If  a  certain  quantity 
of  gas  or  steam  has  to  be  condensed  per  hour,  a  certain  quantity  of 
cooling  water  must  also  be  provided  per  hour,  and  with  certain  forms 
of  condensing  apparatus  the  quantity  of  cooling  water  may  have  to 
be  increased  as  pipe-cleaning  time  recedes,  as  the  deposit  upon  the 
pipes  increases,  and  the  quantity  of  water  may  also  have  to  be 
increased,  owing  to  its  temperature  having  increased  with  the  season 
of  the  year.  If  the  cooling  tower  is  to  be  successful,  the  engineer 
who  has  it  under  his  charge  must  have  sufficient  margin  in  its 
capacity  for  cooling,  to  deal  with  all  these  variations,  and  that  in 
spite  of  the  losses  by  evaporation,  by  wind,  and  other  sources. 


Gas  and  Oil  Power.     Producer  and   Kindred  Gas 

Any  gas  engine  may  be  operated  by  illuminating  gas,  or  by 
producer,  or  other  gas ;  but  the  power  available,  the  effective  horse- 
power, is  approximately  twenty  per  cent,  less  when  producer  gas  is 
employed  than  when  illuminating  gas  is  used.  There  are  two  gases 
that  are  produced  in  industrial  operations  that  for  a  long  time  were 
wasted,  but  which  are  now  gradually  coming  into  use  for  generating 
power — blast  furnace  gas  and  coke-oven  gas.  In  the  process  of  iron 
smelting,  iron  ore,  coke,  and  limestone  are  burnt  together  in  the 
furnace,  with  the  object  of  separating  the  oxygen  and  other  substances 
from  the  metallic  iron  in  the  ore.  The  furnace  is  fed  by  a  blast  of 
air,  forced  in  by  an  engine,  hence  the  name  "  blast  furnace  ; "  and  in 
the  process  a  large  quantity  of  carbonic  oxide  is  formed.  After  the 
iron  has  been  separated  from  its  ore,  there  is  a  large  volume  of  hot 
gas  rising  from  the  top  of  the  furnace,  consisting  very  largely  of  CO. 
In  the  early  days,  and  in  old  furnaces  even  now,  the  gases  may  be 
seen  coming  away  from  the  top  of  the  furnace,  lighting  up  the 
neighbourhood,  but  wasting  a  large  quantity  of  energy.  In  the 
modern  blast  furnace,  a  portion  of  the  heat  of  the  gases  coming  over 
from  the  furnace  is  made  use  of  in  the  "  hot  blast  stoves,"  in  which 
the  air  for  the  blast  is  heated  on  its  way  to  the  furnace,  but  there  is  a 
large  quantity  remaining,  and  in  composition  it  very  closely  resembles 
some  of  the  producer  gases,  its  calorific  value  being  about  130  units 
per  cubic  foot.  Before  it  can  be  used  in  a  gas  engine,  however,  blast 
furnace  gas  must  be  well  cleaned.  With  the  gas  itself  a  large 
quantity  of  dust  comes  over  that  would  be  fatal  to  the  working  of 


PLATE  GA.— The  Ki'irting  Two  Cycle  Gas  Engine,  made  by  Messrs.  Mather  &  Platt 
in  the  United  Kingdom. 


PLATE  GB. — Multipolar  Continuous  Current  Generator  made  by  Messrs.  Mather 
&  Platt.  It  will  be  noticed  that  the  Upper  Half  of  the  Field  Coils  and 
Enclosing  Ring  can  be  lifted  so  that  the  Armature  can  be  got  at. 


[To  face  p.  152. 


THE   GENERATION   OF  ELECTRICITY  153 

any  gas  engine.  There  are  various  methods  of  cleaning  the  gas, 
which  need  not  be  detailed  here,  by  fans  passing  the  gas  through 
water  in  settling  tanks,  and  others.  All  the  difficulties  have  been 
so  satisfactorily  overcome  that  at  several  large  ironworks  the  gas  is 
employed  in  driving  engines  up  to  1000  H.P.  in  this  country,  and  to 
very  much  higher  powers  on  the  Continent.  A  point  that  must 
always  be  looked  out  for  in  all  gas,  except  that  supplied  by  gas 
undertakers  in  towns,  is  the  tarry  compounds  that  are  formed  in  gas 
making.  These  must  always  be  extracted  before  the  gas  is  allowed 
to  enter  the  engine,  or  they  will  lead  to  trouble  with  the  valves,  and 
in  the  cylinder,  as  some  of  the  tarry  matter  is  left  behind  after  the 
exhaust  gases  are  driven  out.  This  will  be  in  addition  to  the  matter 
of  the  dust. 

In  the  process  of  coke  making,  which  is  very  similar  to  that  of 
gas  making,  as  carried  out  in  town  gas  works,  up  to  a  certain  point, 
there  is  a  large  quantity  of  gas  given  off,  which  has  a  much  higher 
calorific  value  than  either  blast  furnace  or  producer  gas,  as  it  is  so 
rich  in  hydrocarbons.  Its  calorific  value  may  be  taken  as  about 
400  heat  units  per  cubic  foot.  A  portion  of  the  gas,  approximately 
half  that  given  off,  is  used  to  heat  the  retorts  in  which  the  coal  is 
being  formed  into  coke,  but  the  remainder  is  available  for  use  either 
for  firing  boilers,  or  in  internal  combustion  engines.  It  is  employed 
at  the  present  time  in  both  ways,  with  very  economical  results. 


Producer  Gas 

Producer  gas  goes  by  several  different  names — water  gas  and 
others — but  all  forms  are  produced  on  some  variation  of  the  one 
method.  Coal  or  coke  is  raised  to  incandescence,  and  steam  or  steam 
and  air  are  driven  through  it,  the  heat  causing  the  steam  to  be 
decomposed  into  its  components,  oxygen  and  hydrogen,  this  being 
followed  by  the  combination  of  both  with  some  of  the  carbon  of  the 
fuel.  The  calorific  value  of  the  different  forms  of  gas  runs  from  130 
heat  units  per  cubic  foot  to  200  heat  units.  In  some  forms  of 
producers  the  generation  of  gas  is  only  one  part  of  the  operation, 
what  are  termed  by-products  being  considered  of  as  much,  if  not  more, 
importance  than  the  gas.  Mond  gas,  of  which  so  much  has  been 
heard  from  time  to  time,  and  of  which  so  much  is  hoped  in  the 
matter  of  distribution  of  power  by  gas,  is  essentially  a  by-product 
process.  There  is  hardly  space  to  go  into  it  here,  but  it  may  be 
mentioned  that  the  process  has  been  very  carefully  and  scientifically 
worked  out,  heat  being  economized  to  the  utmost;  and  the  by- 
products, the  principal  of  which  is  sulphate  of  ammonia,  which  are 
of  considerable  manurial  value,  being  continuously  produced,  gas 


154  ELECTRICITY   IN   MINING 

being  rather  a  by-product  than  the  others.  The  calorific  value  of 
Mond  gas  ranges  from  130  heat  units  to  140  per  cubic  foot.  In 
most  of  the  earlier  producer  plants,  the  generation  of  gas  was  not 
continuous,  a  process  of  changing  over  or  recharging  having  to  be 
gone  through  at  certain  periods,  while  storage  was  necessary  in  some 
cases,  when  the  engines  using  the  gas  were  not  working  continuously 
through  the  twenty-four  hours. 


The  Suction  Gas  Producer 

In  the  latest  apparatus,  however,  which  has  only  been  placed  on 
the  market  in  recent  years,  all  of  these  difficulties  have  been  over- 
come. The  gas  is  produced  as  and  when  it  is  wanted,  its  generation 
being  controlled  by  the  engine  itself.  In  the  suction  apparatus, 
the  draft  necessary  for  keeping  the  furnace  in  which  the  gas  is  being 
generated  in  operation  is  created  by  the  suction  stroke  of  the  engine. 
The  producer  consists  of  an  iron  cylinder,  generally  insulated 
thermally  and  lined  with  firebrick,  in  which  the  fuel  rests  on  a 
grate,  with  an  ashpan  below,  and  with  a  hopper  containing  a  supply 
of  fuel  above.  There  is  a  small  boiler  for  generating  steam,  usually 
in  the  form  of  a  ring,  surrounding  the  top  of  the  furnace.  The 
furnace  is  fed  with  air  from  outside,  and  with  steam  from  the  boiler, 
the  steam  and  air  being  led  together  to  the  bottom  of  the  furnace  by 
pipes  arranged  for  the  purpose.  For  starting  the  apparatus,  a  small 
fan  is  provided,  which  forces  air  through  the  fuel  when  it  is  lighted, 
and  until  sufficient  gas  is  generated  and  enough  heat  to  enable  the 
fan  to  be  dispensed  with.  The  air  and  steam,  as  explained,  combine 
with  the  carbon,  forming  principally  CO  and  COa,  with  a  small 
quantity  of  CH4,  and  the  liberation  of  a  small  quantity  of  free 
hydrogen.  Anthracite  coal  is  the  fuel  preferred  for  the  suction  gas 
producer,  because  it  is  so  rich  in  carbon,  and  coke  is  even  better  if  it 
is  free  from  sulphur ;  but  all  forms  of  coal  may  be  used,  provided 
that  proper  scrubbing  apparatus  is  fixed  in  connection  with  the 
producer.  The  scrubber  consists  of  one  or  more  cylinders,  filled  with 
coke  or  sawdust,  the  former  being  preferable.  Above  the  scrubber 
is  an  arrangement  for  allowing  a  thin,  sprayed  stream  of  water  to 
trickle  constantly  down,  over  and  through  the  coke,  or  the  sawdust. 
The  gas  to  be  scrubbed  enters  the  cylinder  at  the  bottom,  and  passes 
up  through  the  coke  or  the  sawdust,  meeting  the  stream  of  water 
trickling  down,  and  parting  with  all  the  tarry  matters  it  carries,  if 
the  operation  is  properly  carried  out.  It  will  be  evident  that,  within 
certain  limits,  the  operation  of  scrubbing  can  be  carried  as  far  as  you 
please.  The  gas  may  be  subjected  to  the  action  of  as  much  water 
carried  on  the  surface  of  as  much  coke  as  you  like,  and  it  is  only 


THE   GENERATION   OF  ELECTRICITY  155 

necessary  that  the  process  shall  be  carried  far  enough  for  the  gas  to 
come  out  free  of  all  tarry  products.  After  passing  through  the 
scrubber,  the  gas  is  taken  to  what  is  practically  a  receiver,  called  the 
expansion  box,  from  which  it  is  drawn  by  the  engine  at  the  suction 
stroke.  When  the  engine  draws  gas  from  the  receiver,  the  pressure 
there  is  lowered,  and  consequently  the  pressures  at  different  points, 
right  back  to  the  boiler  and  furnace,  air  and  steam  being  then 
supplied  to  the  furnace  in  exact  proportion  to  the  quantity  of  gas 
that  has  been  taken  from  the  receiver.  When  the  engine  stops,  the 
draught  is  automatically  cut  off,  no  air  or  steam  passes  to  the  furnace, 
and  no  gas  is  made.  In  starting  the  producer,  say  in  the  morning, 
the  generation  of  gas  is  tested  from  point  to  point  by  gas  cocks  fixed 
for  the  purpose,  where  the  gas  can  be  burnt  in  a  jet.  It  is  known  by 
a  characteristic  blue  flame,  and  a  smell  of  its  own  that  cannot  easily 
be  mistaken  when  it  has  once  been  experienced.  The  smell  is 
quite  different  from  that  of  ordinary  illuminating  gas.  The  suction 
gas  producer  is  to  a  gas  engine  what  the  boiler  is  to  a  steam  engine, 
with  the  advantage  that  it  requires  very  much  less  attention,  and 
less  fuel  for  stand-by  purposes,  and  it  takes  very  much  less  time 
to  generate  sufficient  gas  to  start  the  engine  than  the  average 
boiler  does  to  make  steam.  Another  advantage  is  claimed  for  the 
suction  apparatus,  viz.  that  the  pressure  within  the  apparatus, 
except  during  the  short  period  in  which  the  fan  is  in  operation,  is 
below  that  of  the  atmosphere,  and  therefore  leakage  is  very  much 
less  likely  to  take  place,  than  with  gas  delivered  under  a  certain 
pressure  from  the  town  supply  service.  It  is  wise,  however,  to 
arrange  that  the  producer  house  is  well  ventilated.  The  suction 
apparatus  also  takes  up  a  comparatively  small  space,  and  requires 
no  chimney,  such  as  is  necessary  with  a  boiler.  From  80  to  100 
cubic  feet  of  producer  or  blast  furnace  gas  is  required  per  brake  horse- 
power per  hour.  With  coke-oven  gas,  a  smaller  quantity  is  require^, 
approximately  in  the  inverse  proportion  to  the  calorific  values  of  the 
different  gases.  It  varies  with  the  fuel  employed  and  other  things,  as 
well  as  with  the  attendance,  so  that  no  absolute  rule  can  be  given.  It 
is  wise,  however,  in  calculations,  to  allow  100  cubic  feet  per  B.H.P. 
for  producer  and  blast  furnace  gases,  and  from  35  to  40  cubic  feet 
with  coke-oven  gas.  The  quantity  of  fuel  consumed  with  the 
producer  gas  is  from  three-quarters  to  one  pound  and  a  quarter 
per  B.H.P.,  and  the  quantity  of  water  required  for  steam,  and 
for  scrubbing  runs  from  1  to  2  gallons  per  B.H.P.  per  hour.  Of 
this,  approximately  one-eighth  to  one-quarter  of  a  gallon  per 
B.H.P.  is  used  for  steam,  and  the  remainder  for  scrubbing  the  gas. 
The  water  required  for  the  latter  can  be  employed  over  and  over 
again,  if  desired,  by  proper  arrangement ;  but  as  any  kind  of  water 
almost  can  be  used,  using  over  and  over  again  is  not  of  importance. 


1 56 


ELECTRICITY  IN   MINING 


THE   GENERATION   OF   ELECTRICITY  157 

Some  suction  plants  are  arranged   to   use  very  little  water  on  the 
scrubbers. 

The  coke  or  sawdust  in  the  scrubber  has  to  be  either  changed  or 
cleaned  periodically,  the  time  depending  upon  the  fuel,  the  water, 
and  the  material  in  the  scrubber.  Fig.  78  shows  a  section  of  a 
Campbell  suction  gas  producer. 


The  Internal  Combustion   Engine 

The  engines  in  which  gas,  and  the  vapour  of  oil  or  petrol  are  used 
to   generate  mechanical  power,  and  which  are  known  as  internal 
combustion   engines,   are   very   different  in  operation    from    steam 
engines.     While  with  steam  engines  it  is  necessary  to  have  a  steam 
boiler  to  generate  the  steam,  that  is  afterwards  employed  in  driving 
the  engine,  with  gas  and  oil  engines  the  whole  of  the  work  is  per- 
formed in  the  cylinder  of  the  engine  itself,  the  mixture  of  gas  and 
air  contained  in  the  cylinder  performing  the  same  office  as  the  steam 
does  in  the  steam  engine.     It  will  be  seen  at  once  what  a  very  con- 
venient arrangement  this  is.     Gas  may  be  generated  miles  away, 
brought  to  the  works  in  pipes,  and  the  necessary  power  created  by 
the  consumption  of  the  gas  in  the  engine  itself.     Or,  per  contra,  gas 
may  be  generated  on  the  ground  and  led  to  the  engine  ;  or,  again,  oil 
may  be  carried  to  the  works  in  any  convenient  manner,  vaporized, 
and  used  in  the  engine.     With  the  exception  of  a  few  of  the  later 
forms,  all  internal  combustion  engines  work  on  what  is  called  the 
Otto  cycle,  and  all  are  constructed  on  very  much  the  same  lines. 
There  is  a  cylinder,  open  at  one  end,  in  which  a  solid  piston  moves 
to  and  fro,  operating  a  connecting  rod  which  communicates  its  motion 
to  the  driving  shaft  of  the  engine.     The  piston  receives  a  violent 
impulse  once  in  every  four  strokes,  which  is  once  in   every  two 
revolutions  of  the  crankshaft.     The  violent  impulse  is  created  by  an 
explosion  of  a  mixture  of  gas,  or  oil  vapour,  and  air.    The  action  is  as 
follows.     Commencing  at  what  is  termed  the  suction  stroke  of  the 
cycle,  the  first  stroke  of  the  piston  outwards,  as  the  piston  moves,  the 
space  left  vacant  is  occupied  by  a  mixture  of  gas  and  air,  valves 
arranged  for  the  purpose,  somewhat  similar  to  the  suction  valves  of 
air  compressors,  being  open  during  that  period,  and  the  extent  to 
which  they  are  open  being  controlled,  in  the  latest  patterns  of  gas 
and  oil  engines,  by  the  work  the  engine  is  performing.     The  power, 
it  will  be  understood,  is  obtained  by  the  combustion  of  the  gases 
which  are  present.     In  ordinary  town  gas  there  is  a  large  percentage 
of  a  carburetted  hydrogen  and  of  hydrogen  gases.     Both  of  these,  the 
carburetted  hydrogen  gas  being  first  decomposed,  combine  readily  with 
the  oxygen  gas  in  the  air  that  is  admitted  with  the  gas,  and  in  doing 


158  ELECTRICITY   IN   MINING 

so  liberate  a  certain  definite  number  of  heat  units.  With  ordinary 
town  gas,  from  600  to  700  B.Th.  Units  are  liberated  per  cubic  foot 
of  the  gas  consumed.  The  heat  so  liberated  expands  the  mixture  of 
gas  and  air  which  remains  after  combustion,  the  expanding  gases 
driving  the  piston  violently  forward.  The  combustion  is  so  rapid 
that  it  has  been  termed  an  explosion,  that  being  the  term  by  which 
we  are  accustomed  to  describe  similar  operations  when  they  occur  in 
coal-mines,  or  in  the  house.  We  have  at  one  instant  a  volume  of  gas 
and  air  which  occupies  a  certain  small  space,  and  at  the  succeeding 
instant  it  tries  to  occupy  a  space  very  many  times  as  great.  In  coal- 
mines and  in  the  house  when  explosions  take  place,  destruction 
follows.  In  the  gas  or  oil  engine  the  piston  gives  way,  moving 
quickly  to  the  front  of  the  cylinder,  and  carrying  the  crankshaft 
round  in  the  process.  After  the  suction  stroke  comes  the  compres- 
sion stroke.  As  the  piston  returns  at  the  completion  of  the  suction 
stroke,  the  gas  and  air  inlet  valves  are  closed,  and  the  gaseous 
mixture,  being  confined  within  the  cylinder,  is  gradually  compressed, 
usually  to  something  like  75  Ibs.  to  the  square  inch.  Compression, 
though  not  absolutely  necessary  with  an  internal  combustion  engine, 
is  of  great  value,  as  it  enables  the  charge  of  gas  to  be  more  completely 
burnt  than  would  be  possible  without,  and  the  higher  pressures 
obtained  enable  a  much  larger  power  to  be  obtained  from  a  given 
size  of  cylinder.  The  molecules  of  the  gases  are  brought  closer 
together,  and  this  facilitates  the  passage  of  the  heat  necessary  for  the 
combustion  of  each  individual  molecule  through  the  mass.  At  the 
commencement  of  the  third  stroke,  the  first  out  stroke  of  the  piston, 
the  mixture  is  ignited,  and  the  explosion  follows.  The  combustion 
of  the  gases  is  not  absolutely  instantaneous.  It  occupies  a  certain 
sensible  period,  measured  by  instruments  that  are  very  sensitive. 
The  combustion  of  the  gases  is  going  on  for  a  large  portion  of  the  out 
stroke,  the  active  stroke  of  the  engine ;  but  the  whole  thing  occupies 
so  short  a  time  that  it  appears  to  be  instantaneous.  The  effect  is,  a 
certain  number  of  heat  units  are  liberated,  according  to  the  quantity 
and  the  composition  of  the  gases,  and  a  certain  portion  of  the  energy 
of  the  heat  is  delivered  to  the  gaseous  mixture  remaining.  On  the 
commencement  of  the  fourth  stroke  of  the  piston,  the  second  return 
stroke,  the  exhaust  valve  is  opened,  and  the  products  of  combustion 
are  forced  out,  giving  rise  to  the  coughing  noise  we  are  so  familiar 
with  where  gas  engines  are  working.  The  ignition  of  the  gaseous 
mixture  is  accomplished  in  the  most  modern  gas  engines  of  small 
size  by  a  hot  tube,  maintained  at  a  high  temperature  by  a  small  jet  of 
gas,  and  exposed  to  the  explosive  mixture  at  the  moment  of  ignition. 
There  is  a  tendency,  however,  in  the  larger  forms  of  gas  engines  to 
adopt  the  electrical  ignition  that  is  common  with  motor  cars.  The 
inlet  and  exhaust  valves  are  worked  by  what  is  called  the  half-time 


THE   GENERATION   OF   ELECTRICITY  159 

shaft,  a  second  shaft  driven  generally  by  bevelled  gear  from  the  main 
crankshaft.  The  half-time  shaft  revolves  once  while  the  crank- 
shaft revolves  twice.  On  the  half-time  shaft  are  cams,  which 
engage  with  levers,  arranged  to  open  the  different  valves  at  the  right 
moment. 


Governing  the   Internal  Combustion   Engine 

The  government  of  the  engine  is  an  important  matter,  and  in  two 
ways.  The  engine,  as  explained,  receives  its  impulse  once  in  every 
two  revolutions.  The  energy  then  delivered  has  to  be  distributed 
over  the  remainder  of  the  cycle.  In  addition  to  this,  if  the  engine  is 
to  work  economically,  some  provision  is  necessary  to  ensure  the  con- 
sumption of  gas  being  approximately  in  proportion  to  the  work  being 
done.  The  first  of  these  objects  is  accomplished  by  the  flywheel. 
All  gas  engines  carry  one,  and  some  carry  two  heavy  flywheels  on 
the  ends  of  the  crankshaft.  The  flywheels  are  proportioned  to  the 
energy  delivered  by  the  explosions ;  but  in  all  cases  they  take  up  a 
sufficient  portion  of  the  energy  of  the  explosion  to  enable  the  piston 
to  perform  its  three  strokes,  during  which  it  receives  no  impulse,  and 
this  notwithstanding  the  work  the  engine  may  be  performing  exter- 
nally at  the  time.  The  result  obtained  is  not  an  absolutely  uniform 
speed,  but  an  average  uniform  speed.  If  counted  for  any  portion  of 
a  cycle,  the  speed  will  be  found  to  vary  considerably ;  but  if  the 
speed  be  taken  for  a  minute,  or  for  successive  minutes,  it  will  be 
found  to  be  very  constant.  When  the  explosion  takes  place  there  is 
an  acceleration  of  speed,  and  this  is  followed  by  a  gradual  slowing 
down  till  the  next  explosion  occurs.  This  last  feature  is  taken 
advantage  of  to  govern  the  engine  with  reference  to  the  load.  The 
half-time  shaft  carries  a  hit-and-miss  governor.  It  is  constructed  on 
the  centrifugal  principle,  as  with  the  steam-engine  governor ;  but,  in 
place  of  opening  a  valve  more  or  less,  as  in  the  steam  engine,  it  either 
opens  the  gas  and  air  valves,  or  does  not  open  them,  according  as  a 
cam  on  the  half-time  shaft  engages  with  a  dog  connected  to  the 
governor  shaft,  or  does  not.  When  the  speed  of  the  engine  has  fallen 
to  a  certain  figure  the  two  engage.  Hence,  if  the  load  is  light,  the 
speed  does  not  come  down  to  the  point,  when  more  gas  and  air  are 
admitted,  until  the  engine  has  made  four,  six,  or  even  eight  revolu- 
tions, while,  when  the  load  is  heavy,  gas  and  air  are  taken  in  at 
every  two.  In  some  of  the  later  forms  of  gas  engine  a  mixing 
chamber  is  provided  into  which  the  gas  and  air  are  admitted,  and 
from  which  they  pass  to  the  engine  cylinder,  under  the  control  of  a 
valve,  which  is  opened  more  or  less  by  a  centrifugal  governor. 


160  ELECTRICITY  IN    MINING 


Cooling  the  Engine  Cylinder 

The  working  of  gas  and  steam  engines  differs  in  another  matter — 
the  temperature  at  which  the  cylinder  walls  are  maintained.  With 
steam  engines,  all  heat  lost  through  the  cylinder  walls  is  a  loss  of 
efficiency,  and  every  effort  is  made  to  prevent  radiation.  With  gas 
and  oil  engines,  the  heat  taken  from  the  cylinder  walls  is  also  loss, 
but  it  is  necessary  to  enable  the  engine  to  continue  working.  The 
temperatures  created  by  the  explosions  are  very  high,  and  a  large 
portion  of  the  heat  liberated  necessarily  passes  to  the  cylinder  walls 
and  thence  to  the  castings,  of  which  they  form  a  part,  and  in  which 
the  valves  are  fixed.  Hence,  if  the  temperature  of  the  mass  of  the 
casting  is  allowed  to  rise  above  a  certain  figure,  the  mixture  of  gas 
and  air  may  be  fired  immediately  it  is  admitted,  and  then  the  power 
obtained  would  be  small.  Hence  arrangements  are  made  to  carry  off 
a  large  portion  of  the  heat,  by  causing  a  stream  of  water  to  circulate 
round  the  cylinder.  The  walls  of  the  cylinder  are  cast  with  a  hollow 
jacket,  which  sometimes  extends  to  the  back  of  the  cylinder  in 
which  the  valves  are  fixed.  Tanks  of  water  are  provided,  placed 
in  any  convenient  position  near  the  engine,  and  connected  to  the 
water  jacket  by  pipes.  Usually  the  heat  delivered  to  the  water  in 
the  jacket  is  sufficient  to  give  the  required  circulation.  The  top  of 
the  water  jacket  is  connected  to  the  top  of  the  water  tanks,  and  the 
bottom  of  the  water  tanks  to  the  underside  of  the  jacket.  The  hotter 
water  flows  to  the  upper  side  of  the  jacket,  and  thence  to  the  tanks, 
while  the  cooler  water  from  the  bottom  of  the  tanks  passes  to  the 
underside  of  the  jacket.  The  circulation  goes  on  as  long  as  there  is 
an  appreciable  difference  between  the  temperatures  of  the  water  in 
the  jacket  and  that  in  the  tanks.'  In  hot  climates  trouble  has 
arisen  from  the  high  temperature  of  the  only  water  obtainable,  but  it 
has  been  overcome  by  increasing  the  quantity  of  water  in  the  tanks. 
The  tanks  are  generally  made  in  the  form  of  galvanized  iron 
cylinders,  holding  a  certain  number  of  gallons  as  required.  The 
number  of  cylinders  required  is  also  very  easily  calculated.  The 
heat  that  is  to  be  carried  off  by  the  water  in  the  case  of  each  gas  or 
oil  engine  per  minute  is  known.  The  initial  temperature  of  the 
cooling  water  being  known,  and  the  temperature  to  which  it  may  be 
raised  before  its  cooling  effect  becomes  too  small  to  keep  the  engine 
running,  a  simple  calculation  will  give  the  number  of  gallons  of 
water  required  for  any  given  working  day,  and  the  number  of  tanks. 
The  cooling  effect  of  the  water  may  be  assisted,  where  water  is  scarce, 
by  passing  it  through  cooling  towers  or  equivalent  apparatus,  as 
explained  on  p.  146. 


PLATE  TA.— Enclosing  Ring,  Field  Magnet  Coils,  Pole  Pieces,  and  Brush 
Gear  of  a  Multipolar  Continuous  Current  Generator,  by  the  General 
Electric  Co. 


PLATE  7s.— Motor  Generator  made  by  Messrs.  J.  H.  Holmes  &  Co 
The  Machine  on  the  Left  is  a  Three  Phase  Induction  Motor,  that  on 
the  Right  a  Continuous  Current  Generator. 

[To  face  p.  160. 


THE   GENERATION   OF   ELECTRICITY  161 


Gas  Engines  for  Large  Powers 

The  Otto  cycle,  with  one  or  more  cylinders  working  together,  can 
be  employed  for  large  powers,  but  the  sizes  of  the  engines  tend  to 
become  large  for  powers  such  as  those  that  have  been  named, 
1000  H.P.  and  2000  H.P.,  and  so  attempts  have  been  made  to  bring 
the  gas  engine  nearer  the  steam  engine.  These  attempts  have  been 
made  principally  on  the  Continent  of  Europe,  where  the  lead  has 
been  taken  in  the  use  of  blast  furnace  gas  for  power.  In  one  form  of 
engine  the  Otto  cycle  is  employed,  but  two  cylinders  are  arranged 
tandem  with  their  pistons  on  one  rod,  and  delivering  their  power 
to  one  crank.  The  inlet  and  exhaust  ports  of  the  two  cylinders 
are  fixed  in  the  ends  of  the  cylinders  farthest  removed  from  each 
other,  at  opposite  ends  of  the  cylinder  system,  in  fact. 

The  different  parts  of  the  cycle  are  going  on  oppositely  in  the 
two  cylinders.  Thus,  calling  the  two  cylinders  A  and  B,  when 
cylinder  A  is  taking  in  gas,  cylinder  B  is  compressing,  and  when 
cylinder  A  is  compressing,  cylinder  B  takes  an  impulse.  When 
cylinder  B  exhausts,  cylinder  A  explodes,  and  so  on.  By  arranging 
two  pairs  of  cylinders,  each  pair  tandem,  on  opposite  sides  of  the 
flywheel,  an  impulse  every  stroke  is  obtained ;  and  this  method  has 
been  employed  for  driving  electric  alternate  current  generators  that 
have  to  run  together  in  synchronism,  with  considerable  success.  But 
an  advance  has  been  made  upon  this  in  the  Korting  and  Oechelhausen 
and  other  engines,  in  which  each  cylinder  is  double-acting,  while 
pairs  of  cylinders  may  be  arranged  tandem,  and  two  pairs  of  tandems 
on  opposite  sides  of  the  flywheel.  In  this  form  of  engine,  both 
cylinder  ends  are  closed,  as  in  a  steam  cylinder,  the  exhaust  port 
being  in  the  middle  of  the  cylinder,  and  uncovered  by  the  piston. 
Gas  and  air  are  taken  in  at  each  stroke,  just  as  gas  is  in  the  double- 
acting  compressor,  the  inlet  valve  being  closed  at  a  certain  portion  of 
the  stroke,  after  which  compression  commences.  The  outstroke  of 
the  piston  with  this  form  of  gas  engine  is  the  explosion  stroke,  the 
return  stroke  being  both  the  charging  stroke  and  the  compression 
stroke.  There  is  no  suction  stroke,  the  charging  of  the  cylinders 
being  performed  by  a  pair  of  pumps,  one  for  gas  and  the  other  for 
air.  The  pumps  are  so  arranged  that  the  gas  and  air  are  always 
admitted  in  the  proper  proportions,  under  a  pressure  of  about  9  Ibs. 
per  square  inch,  and  so  that  a  scavenging  current  of  air  is  driven 
into  the  cylinder  after  the  exhaust  gases  have  passed  out,  before  the 
fresh  charge  of  gas  and  air  is  admitted.  The  piston  is  made  especially 
long.  The  cycle  may  be  taken  as  follows:  Commencing  with  the 
'explosion  at  one  end  of  the  cylinder,  the  piston  is  driven  forward  to 

M 


162 


ELECTRICITY  IN   MINING 


the  other  end  by  the  expan- 
sion of  the  burnt  gases,  inlet 
and  compression  proceeding 
at  the  other  end  of  the  piston. 
As  the  exhaust  port  is  un- 
covered, the  exhaust  gases 
commence  to  escape.  Then 
comes  the  current  of  air, 
clearing  out  the  remaining 
products  of  combustion,  and 
clearing  the  passages  ;  then 
the  inlet  valve  is  opened  for 
the  admission  of  the  fresh 
charge  of  gas  and  air,  and 
meanwhile  the  charge  at  the 
other  end  has  exploded,  and 
the  piston  comes  back,  the 
exhaust  having  been  closed 
in  its  passage.  Then  the 
inlet  valve  is  closed,  com- 
pression commences,  con- 
tinuing till  the  end  of  the 
stroke,  ignition  taking  place 
at  the  commencement  of 
the  out  stroke,  and  so  on. 
In  some  engines  an  extra 
cylinder  is  added  to  the 
plant,  the  duty  of  which 
is  to  scour  the  working 
cylinders  after  the  exhaust 
gases  have  escaped.  The 
scavenging  cylinder,  as  it  is 
called,  has  its  own  piston 
worked  from  the  crankshaft, 
which  compresses  air  in  the 
cylinder,  the  compressed  air 
being  turned  into  the  work- 
ing cylinders  during  the 
scavenging  period.  The 
Korting  gas  engine  is  stated 
to  use  92  cubic  feet  of  gas, 
having  a  calorific  value  of 
110  B.Th.  Units  per  cubic 
foot  per  B.H.P.  Fig.  79 
shows  a  section  of  the 


THE   GENERATION   OF   ELECTRICITY  163 

Oechelhausen  gas  engine ;  Plate  5,  an  Oechelhausen  gas  engine  driving 
a  continuous  current  muitipolar  dynamo;  and  Plate  6A  shows  a 
complete  Korting  gas  engine. 


Oil   Engines 

As  already  explained,  the  oil  engine  is  really  a  gas  engine.  It 
burns  a  gas  made  from  oil,  and  it  is  practically  the  same  as  the  gas 
engine  in  every  respect,  except  that  provision  has  to  be  made  for 
converting  the  oil  into  vapour.  In  the  petrol  engine,  which  is  so 
much  used  in  motor  cars,  and  which  is  also  an  oil  engine,  the  apparatus 
which  converts  the  oil  into  vapour  is  called  a  "  carburetter."  In  the 
stationary  oil  engine  it  is  called  a  "vaporizer."  There  are  two 
methods  of  vaporizing  employed — the  application  of  heat,  causing  the 
liquid  to  evaporate  in  the  usual  way,  and  spraying.  In  the  latter 
method  the  oil  is  subjected  to  the  action  of  compressed  air  or  some 
equivalent  arrangement,  the  oil  being  broken  up  into  a  fine  spray. 
The  object  in  both  methods  is  to  produce  a  fine  state  of  division  of 
the  oil,  so  that  it  can  mix  with  the  air  in  the  same  manner  as  coal  or 
producer  gas  does.  In  the  modern  oil  engine  the  two  methods — 
spraying  and  heating — are  combined,  the  oil  being  sprayed  into  the 
vaporizer  chamber,  which  is  heated.  In  the  petrol  motor  engine, 
the  carburetter  is  a  separate  device,  carburation  taking  place  before 
the  mixture  of  vapour  and  air  enters  the  engine  cylinder ;  but  in  the 
stationary  oil  engine  the  "  vaporizer  "  forms  part  of  the  engine  itself 
— in  many  forms  an  extension  of  the  engine  cylinder.  In  Messrs. 
Hornsby's  oil  engine,  the  vaporizer  is  a  chamber  at  the  back  of  the 
cylinder,  connected  with  the  cylinder  by  a  small  passage.  On  the 
suction  stroke  of  the  engine,  air  only  is  drawn  into  the  cylinder,  oil 
being  at  the  same  time  sprayed  into  the  vaporizer.  At  a  certain 
stage  of  the  compression  stroke  the  compressed  air,  having  become 
heated  to  a  certain  temperature,  is  forced  into  the  vaporizer  chamber, 
where  it  meets  the  oil  vapour,  mixes  with  it,  and  the  whole  being  at 
a  sufficient  temperature,  the  mixture  ignites  at  the  end  of  the  com- 
pression stroke,  the  explosion  following,  as  in  the  gas  engine.  In 
the  Hornsby  engine,  the  vaporizer  is  heated  by  the  exhaust  gases, 
which  are  made  to  pass  out  in  its  neighbourhood,  and  by  the  general 
heating  of  the  engine  body. 

The  spraying  apparatus  consists  of  what  is  called  a  needle 
valve,  an  arrangement  something  similar  to  a  steam  injector.  It 
has  a  fine  tube  passing  into  the  vaporizer  chamber,  the  tube  being 
partly  filled  with  a  fine  needle  the  position  of  which  can  be  regu- 
lated according  to  the  kind  of  fuel  and  the  rate  at  which  vapori- 
zation is  to  go  on.  The  vaporizer  is  also  capable  of  alteration  for 


164  ELECTRICITY  IN   MINING 

different  kinds  of  fuel  by  altering  a  portion  of  the  fittings.  When 
starting,  a  charge  of  vapour  is  formed  by  the  aid  of  a  lamp  provided 
for  the  purpose,  fed  with  the  same  oil  as  the  engine  uses.  The  pre- 
liminary heating  of  the  vaporizer  chamber  takes  from  seven  to  ten 
minutes,  the  chamber  being  heated  to  a  dull  red.  The  lamp  is  then 
turned  off,  and  is  not  required  again  till  the  engine  is  restarted. 
The  reservoir  of  oil  is  kept  in  the  casting  upon  which  the  engine 
stands,  and  it  is  fed  to  the  spraying  nozzle  by  a  small  pump.  In 
the  National  Gas  Co.'s  engine,  which  is  very  much  on  the  lines  of 
the  Hornsby,  the  parts  are  nearly  the  same,  except  that  an  ignition 
tube  is  held  in  the  rear  end  of  the  vaporizer  chamber,  and  that  a 
special  device  is  added  for  conveying  a  jet  of  hot  compressed  air  from 
the  cylinder  to  the  neighbourhood  of  the  hot  tube.  The  front  of  the 
vaporizer  chamber  is  recessed,  and  the  rear  of  the  piston  is  cut  away 
to  fit  the  recess,  so  that  when  the  compression  stroke  is  complete,  the 
piston  enters  this  recess.  Communicating  with  the  rear  end  of  the 
cylinder  outside  the  recess  is  a  small  passage  leading  to  the  space  in 
front  of  the  ignition  tube,  and  as  the  piston  returns,  the  air  is  forced 
along  this  passage.  On  meeting  the  mixture  of  vapour  and  air  in 
front  of  the  tube,  the  temperature  of  the  whole  is  raised  sufficiently 
for  explosion.  In  both  the  Hornsby  and  the  National,  and  in  all 
the  engines  worked  on  this  method,  the  combustion — which  is  com- 
menced at  the  rear  end  of  the  vaporizer — passes  onward  to  the 
remainder  of  the  compressed  charge,  liberating  heat,  and  causing 
expansion  of  the  products  of  combustion,  as  in  the  gas  engine.  In 
the  Campbell  and  others  the  oil  is  carried  in  a  tank  above  the  cylinder, 
and  runs  by  gravity  into  the  vaporizer.  In  these  forms  the  air  drawn 
in  on  the  suction  stroke  is  made  to  spray  the  oil  into  the  vaporizer, 
the  latter  being  a  hot  chamber,  whose  walls  immediately  convert  the 
finely  divided  oil  particles  into  vapour,  which  mixes  with  the  air 
which  has  formed  it.  From  the  vaporizer,  in  these  forms,  the  mixture 
of  vapour  and  air  passes  into  the  engine  cylinder,  where  it  is  com- 
pressed in  the  usual  way,  and  fired  by  a  hot  tube  at  the  rear  end,  on 
the  finish  of  the  compression  stroke.  The  ignition  tube,  in  these 
forms,  is  sometimes  heated  by  a  lamp,  and  sometimes  not,  the  pro- 
vision of  a  lamp,  in  the  case  of  the  Campbell  engine,  being  apparently 
a  matter  of  precaution,  as  in  one  of  the  tests  recorded  it  is  stated  that 
the  lamp  was  not  in  use  the  greater  part  of  the  time.  Broadly,  it 
may  be  taken  that  a  lamp  is  necessary  with  all  oil  engines  using 
ignition  tubes,  for  starting,  in  case  of  the  tube  cooling ;  but  in  all 
cases  the  whole  mass  of  the  engine  becomes  sufficiently  hot,  after 
running  a  short  time,  to  keep  the  tube  at  the  required  tempera- 
ture. The  Campbell  Co.  have  recently  added  to  their  apparatus  the 
provision  of  a  small  jet  of  water,  which  enters  the  engine  cylinder 
with  the  vapour  and  air,  and  they  state  that  they  obtain  a  further 


THE   GENERATION    OF  ELECTRICITY  165 

economy  in  oil  consumption  by  its  use.  The  office  of  the  cooling 
spray  is  to  keep  the  engine  cylinder  cool  by  absorbing  heat  for  its 
conversion  into  steam.  The  steam,  when  formed,  also  adds  to  the 
push  given  to  the  piston  by  its  own  expansion.  The  use  of  water  in 
this  manner  is  coming  in  in  several  cases. 


Governing  the  Oil  Engine 

Several  forms  of  oil  engines  are  governed  simply  on  the  hit-and- 
miss  principle,  described  in  connection  with  gas  engines,  the  supply 
of  oil  vapour  being  cut  off  when  the  speed  of  the  engine  exceeds  a 
certain  figure.  This  is  largely  the  method  employed  with  petrol 
engines  used  for  motor  cars.  But  in  many  forms  of  the  stationary 
oil  engine,  an  attempt  has  been  made  to  obtain  better  government, 
and  to  proportion  the  consumption  of  oil  to  the  engine,  in  accordance 
with  the  load,  more  on  the  lines  of  the  steam  engine.  In  some  forms 
the  governor  controls  the  speed  of  the  pump  that  feeds  the  oil  to  the 
vaporizer,  lessening  the  supply  with  the  increased  speed,  and  vice 
versa.  In  other  forms,  those  in  which  the  oil  runs  down  by  gravity 
to  the  vaporizer,  the  governor  controls  a  graduated  valve,  through 
which  the  oil  passes  to  the  vaporizer,  closing  it  partially  when  the 
speed  increases,  and  vice  versa.  In  the  Hornsby  engine  the  pump 
continues  to  deliver  the  same  quantity ;  but  if  the  speed  rises  above 
a  certain  figure,  the  surplus  oil  is  returned  to  the  tank.  In  the  Camp- 
bell engine  the  governor  pushes  down  a  steel  catch  when  the  speed 
exceeds  a  certain  figure,  preventing  the  exhaust  valve  closing.  As 
this  prevents  the  necessary  lowered  pressure  being  formed  in  the 
engine  cylinder  and  its  adjuncts,  no  air  is  sucked  in,  and  therefore 
no  oil  passes  into  the  vaporizer.  It  will  be  understood  that  in  some 
of  these  patterns,  those  in  which  the  oil  runs  down  by  gravity,  the 
passage  of  the  air  is  necessary  to  bring  the  oil  into  the  vaporizer. 
There  is  an  injector  action  in  connection  with  the  air  and  the  oil,  the 
passage  of  the  air  in  front  of  the  tube  drawing  the  oil  out  into  the 
passage  leading  to  the  vaporizer.  The  action  is  similar  to  that  of 
the  scent  spray,  and  it  operates  very  frequently  in  ventilation. 


The  Ignition  Problem 

With  petrol  motor-car  engines  ignition  has  settled  down  com- 
pletely to  the  electric  spark,  and  this  appears  to  be  preferred  also 
by  a  few  makers  of  stationary  oil  engines ;  but  the  great  majority 
prefer  the  ignition  tube.  The  reason  given  by  one  maker  is,  the 


1 66  ELECTRICITY   IN   MINING 

platinum  points  between  which  the  ignition  spark  passes  become 
clogged  with  a  mass  of  carbon,  which  prevents  the  passage  of  the 
spark.  There  is  a  great  deal  of  truth  in  this.  In  petrol  motor-car 
engines  there  is  some  trouble  from  this  cause,  and  petrol  is  much 
less  liable  to  deposit  carbon  than  the  heavier  oils  that  are  used  in 
stationary  engines.  The  arrangement  for  ignition  by  the  electric 
spark  is  as  follows.  Some  form  of  terminal  piece  is  fixed  in  the 
cylinder  or  vaporizer,  where  it  is  desired  that  combustion  shall 
commence.  There  are  two  forms  of  sparking  arrangements.  The 
most  common  consists  of  a  porcelain  plug,  fixed  in  a  screwed  metal 
fitting,  which  is  screwed  into  the  cylinder.  The  plug  carries  at  its 
inner  end  two  small  platinum  wires,  insulated  from  each  other, 
placed  with  their  ends  at  such  a  distance  apart  that  the  spark,  a 
"  fat  spark,"  will  pass  easily  across.  The  current  for  this,  which  is 
at  a  pressure  of  several  thousands  of  volts,  is  provided  by  a  battery 
of  accumulators,  or  dry  cells,  and  an  induction  coil.  Dry  cells  are 
going  out  rapidly  for  motor-car  work,  as  they  are  so  uncertain ; 
while  for  stationary  work,  where  it  is  not  convenient  to  charge 
accumulators,  bichromate  cells  may  be  used.  The  apparatus  is 
completed  by  what  is  called  the  commutator,  which  usually  consists 
of  a  disc  of  insulating  material,  carrying  contact  pieces,  either  on 
its  edge  or  on  its  face  near  the  edge,  with  a  contact  held  by  a  spring 
pressing  against  the  disc.  The  disc  is  revolved  by  the  half-time 
shaft,  and  when  one  of  its  contact  pieces  comes  opposite  the 
stationary  contact  piece,  the  circuit  is  closed,  and  is  broken  imme- 
diately afterwards,  as  the  disc  moves  on,  a  spark  then  passing 
between  the  points  in  the  cylinder,  the  commutator  arranging  that 
the  spark  passes  at  the  time  it  is  required  to  explode  the  charge. 
With  the  other  method,  known  as  the  magneto,  a  small  magneto- 
electric  machine  takes  the  place  of  the  battery  and  induction  coil ; 
its  armature,  which  is  made  on  various  patterns,  being  operated  by 
gearing  from  the  half-time  shaft.  There  is  no  commutator,  but  in 
its  place  the  half-time  shaft  works  a  rod  which  breaks  a  contact 
inside  the  cylinder. 


The  Diesel  Engine 

The  Diesel  is  also  an  oil  engine,  but  on  very  novel  lines.  There 
is  no  ignition  tube  nor  equivalent  device,  the  ignition  being  accom- 
plished by  the  heat  generated  in  the  air,  which  is  compressed  in  the 
cylinder  for  the  purpose,  and  which  combines  with  the  oil  vapour. 
In  the  Diesel  engine  there  are  practically  three  operations  going  on. 
Air,  only  without  any  vapour,  is  sucked  into  the  cylinder  on  the 
suction  stroke — the  Diesel  engine  works  on  the  Otto  cycle — and  is 


THE  GENERATION   OF  ELECTRICITY 


167 


compressed  on  the  return  stroke  to  a  pressure  of  500  Ibs.  per  square 
inch,  giving  a  temperature  of  approximately  1000°  Fahr.  At  the 
same  time,  the  engine  is  driving  a  two-stage  air  compressor  by 
gearing  from  its  crankshaft,  supplying  compressed  air  to  a  reservoir, 
which  is  maintained  at  from  750  Ibs.  to  800  Ibs.  pressure.  At  the 
moment  when  ignition  takes  place  in  the  ordinary  type  of  gas  and 
oil  engine,  a  very  finely  divided  spray  of  oil  is  injected  into  the 


FIG.  80.— Sections  of  Diesel  Oil  Engine. 

cylinder,  and  sprayed  by  means  of  the  compressed  air  in  the  reservoir. 
The  oil  spray,  meeting  the  air  in  the  cylinder  at  the  high  temperature 
mentioned,  burns,  the  oxygen  necessary  for  its  combustion  being 
provided  by  the  air  which  ignites  it.  The  oil  is  said  to  burn  steadily, 
the  effect  being  more  like  that  of  steam  entering  a  steam  engine  than 
the  explosive  force  of  the  gas  and  oil  engines  described.  The  oil 
is  forced  into  the  cylinder  by  means  of  a  pump,  as  in  some  of  the 


1 68  ELECTRICITY   IN   MINING 

other  oil  engines,  the  compressed  air  assisting  it  to  drive  it  in,  and 
atomizing  it  in  the  process.  The  remainder  of  the  cycle  is  the  same 
as  with  other  oil  engines.  Any  kind  of  oil  may  be  used  in  the 
Diesel  engine,  but  the  cheaper  kinds,  the  crude  heavy  carbon  oils, 
are  preferred,  because  they  are  cheaper  and  richer  in  carbon.  The 
cost  for  fuel,  when  the  crude  oils  are  employed,  is  claimed  to  be 
less  than  that  of  any  other  oil  engine.  The  quantity  per  B.H.P.  is 
rather  less  than  half  a  pint,  while  the  cost  of  the  crude  substance 
is  much  less  than  the  oil  sold  for  illuminating  purposes.  The  crude 
oil  is  the  refuse  after  all  the  refined  oils  have  been  distilled  off. 
The  fuel  cost,  using  this  substance,  is  stated  to  be  about  one-tenth 
of  a  penny  per  B.H.P.  The  Diesel  engine  is  governed  by  controlling 
the  pump  supplying  oil  to  the  cylinder. 

The  Diesel  Co.  claims,  however,  that  they  obtain  a  government 
much  nearer  the  load  than  is  possible  with  the  usual  type  of  oil 
engine,  inasmuch  as  the  quantity  of  oil  allowed  to  enter  the  cylinder 
is  arranged  at  the  very  last  minute,  just  before  combustion  com- 
mences, and  they  show,  by  the  aid  of  indicator  diagrams,  that  the 
effect  produced,  when  the  engine  is  working  at  less  than  full  load, 
is  very  similar  to  that  in  a  steam  engine  whose  governor  controls 
the  slide  cut-off.  In  fact,  as  mentioned  above,  the  Diesel  engine 
approaches  very  closely  in  its  working  stroke  to  the  steam  engine. 
Fig.  80  shows  sections  of  one  form  of  the  Diesel  engine. 


A  Coal-dust  Burning  Engine  on  Similar 
Lines  to  the  Diesel 

It  will  be  of  interest  to  mention  that  attempts  have  been  made 
to  work  engines  on  the  same  principle  as  the  Diesel  oil  engine,  with 
coal  dust  as  a  fuel.  Coal  dust  is  being  introduced  for  firing  steam 
boilers,  on  the  same  lines  as  oil  fuel,  and  it  is  a  natural  extension 
of  the  principle  to  work  internal  combustion  engines  with  the  same 
fuel.  At  the  Glasgow  Exhibition  of  1901  an  engine  was  exhibited, 
though  the  writer  believes  it  was  not  actually  run  in  the  exhibition, 
of  150  H.P.,  in  which  coal  dust  was  the  fuel.  The  coal  dust  was 
injected  into  the  cylinder  in  the  same  manner  as  the  vapour  of  oil 
in  the  Diesel  engine,  the  charge  being  ignited  by  the  heated  air,  which 
had  previously  been  compressed  in  the  same  manner  as  in  the  Diesel 
engine.  The  engine,  the  writer  believes,  worked  on  the  Otto  cycle. 
The  air  for  the  engine  was  warmed  before  passing  into  the  cylinder 
by  passing  over  parts  of  the  engine  which  were  at  a  high  temperature. 
The  engine  has  not  been  placed  on  the  market.  Possibly  difficulties 
arose  in  connection  with  its  working  when  in  service  that  had  not 
disclosed  themselves  in  the  experimental  stage;  but  there  would 


PLATE  SA. — Armature  of  Three  Phase 
Generator  for  Turbo  Driving  ready  for 
the  Coils,  made  by  Dick,  Kerr  &  Co. 


PLATE  SB. — Armature  of  Three  Phase 
Generator  for  Turbo  Driving,  with  Coils 
complete,  made  by  Dick,  Kerr  &  Co. 


PLATE  8c.— Revolving  Field  Magnets  of  Three  Phase  Alternator  for  Direct 
Driving  from  a  Steam  Turbine,  made  by  Messrs.  Dick,  Kerr  &  ^Co.  The 
Rings  shown  on  the  Axle  are  to  deliver  the  Current  to  the  Field  Coils. 


[To  face  p.  168. 


THE   GENERATION    OF   ELECTRICITY  169 

appear  to  be  no  reason,  other  than  practical  ones,  for  its  non-success. 
Every  one  who  has  visited  a  colliery  where  screening  is  going  on  is 
familiar  with  the  cloud  of  finely  divided  particles  present  in  the  air, 
and  it  is  these  that  are  made  to  ignite  in  the  engine  cylinder,  mixing 
with  the  air  necessary  for  combustion,  in  the  same  way  as  the  vapour 
of  oil  does. 

Having  decided  on  the  source  of  power,  the  engines  to  be  used  for 
driving  the  dynamos,  the  next  items  in  the  generating  station  are 
the— 


Generators  of  Electricity 

Two  forms  of  generators  of  electricity  are  now  employed  in  mining 
work,  for  continuous  current  and  for  three-phase  alternating  current. 
Single-phase  alternating  current  is  not  yet  suitable  for  mining  work, 
because  the  single-phase  motor  is  not  yet  a  practical  machine  such  as 
could  be  employed  for  driving  mining  machinery.  The  two-phase 
generator  is  almost  the  same  as  the  three-phase  generator.  The 
principle  upon  which  continuous  current  and  alternating  current 
generators  are  based  is  the  same.  When  a  conductor  is  moved  through 
a  magnetic  field,  or  when  the  strength  of  the  magnetic  field  in  which 
a  conductor  is  lying  is  changed,  or  when  any  equivalent  of  this  is 
produced,  an  electric  pressure  is  created  in  the  conductor,  propor- 
tional to  the  strength  of  the  magnetic  field,  and  to  the  rate  at  which  the 
change,  in  its  bearing  upon  the  conductor,  takes  place.  Put  in  another 
form4  assuming  the  conductor  to  be  in  the  form  of  a  loop,  as  all  con- 
ductors used  for  generating  current  are,  the  pressure  created  depends 
directly  upon  the  rate  of  change  of  the  number  of  lines  of  force  passing 
through  the  loop.  In  practical  dynamo  machines  there  are  a  number 
of  loops  of  conductors,  and  they  are  held  sometimes  in  slots  on  the 
peripheries  of  drums  built  up  of  thin  iron  or  steel  plates,  sometimes  in 
slots  in  discs  held  on  the  inside  of  iron  or  steel  cylinders.  In  both 
cases  it  is  arranged  that  a  powerful  magnetic  field  is  created  within  a 
small  annular  space  between  an  outer  cylinder  and  an  inner  one.  In 
continuous  current  machines  the  usual  arrangement  is,  there  is  an 
outer  cylinder  of  iron  or  steel  having  feet  for  fixing  to  foundations,  or 
forming  part  of  a  bedplate  which  performs  the  same  office.  Held  on 
the  inside  of  the  cylinder,  and  pointing  radially  inwards,  are  cores  of 
electro-magnets,  the  cores  being  sometimes  of  wrought  iron  or  mild 
steel,  forming  part  of  and  cast  with  the  containing  cylinder,  some- 
times built  up  of  thin  iron  plates  cast  into  the  steel  or  iron  cylinders, 
when  the  latter  is  cast,  and  sometimes  of  other  arrangements.  The 
magnetic  field  in  the  polar  space  on  the  inside  of  the  cores  of  the 
field  magnets  is  created  by  currents  passing  in  coils  of  wire  round 


170  ELECTRICITY   IN   MINING 

the  radial  cores  described  above.  In  modern  machines  the  field- 
magnet  coils  are  wound  sometimes  on  wood  spools,  sometimes  on 
metal  spools,  and  sometimes  are  simply  made  up  into  coils.  In 
either  case  they  are  very  carefully  insulated,  the  wires  of  which  the 
coils  are  composed  having  all  the  moisture  extracted  from  their  cotton 
coverings,  in  a  vacuum  oven,  and  being  afterwards  steeped  in  an 
insulating  varnish  which  resists  moisture  and  heat,  and  then  wrapped 
with  insulating  tapes,  and  in  other  ways  protected  from  mechanical 
injury,  damp,  etc.  It  is  a  common  practice  to  utilize  the  crescent- 
shaped  ends  of  the  cores  of  the  field  magnets  to  hold  the  field  coils  in 
position.  Where  the  field  magnet  cores  are  cast  in  the  containing 
ring,  the  formed  coils  are  slipped  over  them,  and  the  pole  pieces  are 
then  fixed  outside  of  the  field  coils,  and  bolted  to  the  cores.  The 
crescent-shaped  pole  pieces  form  an  inner  cylinder  broken  by  the 
gaps  between  them.  Plate  GB  shows  a  complete  multipolar  con- 
tinuous current  generator,  made  by  Messrs.  Mather  &  Platt. 


The  Armature 

In  the  continuous  current  machine  the  armature,  on  which  the 
wires  that  are  to  perform  the  office  of  generating  the  current  are 
carried,  is  built  up  of  very  thin  iron  or  steel  plates.  In  the  smaller 
machines  the  plates  are  in  the  form  of  complete  discs.  In  the  larger 
machines  they  are  in  the  form  of  sectors  of  discs,  there  being  as  many 
sectors  as  there  are  pairs  of  poles  in  the  machine.  In  both  cases  the 
peripheries  of  the  plates  are  slotted,  the  inner  edges  are  punched  or 
slotted,  and  the  plates  are  built  up  upon  brass  spiders,  themselves  made 
in  sections  in  multipolar  machines,  the  whole  being  held  upon  a 
brass  boss  in  the  centre,  through  which  the  driving  axle  of  the  machine 
passes,  and  to  which  it  is  keyed.  The  peripheral  slots  on  the  plates, 
when  the  latter  are  in  position,  form  longitudinal  channels  in  which 
the  copper  conductors  lie.  Before  building  into  the  drums  the  iron 
plates  form,  they  are  first  varnished.  The  modern  plan  is,  the  plates 
pass  through  varnishing  machines  consisting  of  rollers  over  which 
varnish  drips,  the  varnish  being  spread  out  over  each  plate  in  a  thin 
layer.  The  plate  is  then  carried  on  to  a  drying  apparatus,  drying 
being  accomplished  by  hot  air,  and  the  plate  emerging  at  the  opposite 
end  of  the  machine,  after  a  few  minutes,  with  a  dry  adherent  coating 
of  varnish  on  each  side.  In  some  machines  a  very  thin  sheet  of  paper 
is  placed  between  the  plates.  In  all  large  machines,  also,  air  ducts 
are  provided  at  certain  portions  of  the  lengths  of  the  drum  by  fixing 
distance  pieces  between  successive  sections  of  plates,  the  air  passing 
through  the  centre  of  the  drum,  and  out  at  the  periphery  through  the 
spaces  left  between  the  plates.  When  the  plates  are  assembled  on 


THE   GENERATION   OF   ELECTRICITY  171 

their  spiders,  they  are  squeezed  together  by  hydraulic  pressure,  and 
retained  in  position  by  massive  iron  end  discs.  The  conductors  for 
continuous  current  machines  are  almost  invariably  what  is  called 
"  former  wound."  The  coil  is  made  on  a  wooden  former  of  the  exact 
shape  the  coil  will  take  when  it  is  in  position  on  the  armature.  After 
being  formed  it  is  dried  in  the  vacuum  oven,  varnished,  the  varnish  set 
by  heat  in  the  oven,  and  it  is  then  protected  by  insulating  tapes,  which 
are  also  dried  and  varnished,  the  varnish  being  dried  by  heat,  the 
ends  of  the  coil  being  left  out  in  the  positions  they  are  to  occupy.  The 
coils  are  carefully  tested  before  being  placed  in  the  machine,  for 
resistance,  each  coil  being  exactly  like  all  the  others  on  the  same 
machine.  The  longitudinal  channels  on  the  armature  are  carefully 
milled  out,  all  pin  points,  iron  dust,  etc.,  removed,  and  the  channels 
themselves  are  lined  sometimes  with  mica  built  into  the  form  of  the 
channel,  sometimes  with  micanite,  a  substance  formed  by  building  the 
thin  shreds  of  mica  into  a  flexible  cloth  or  sheet,  and  sometimes  with 
other  substances,  such  as  presspahn.  It  will  be  understood  that  the 
possibility  of  a  connection  between  the  coil  and  the  iron  in  which  it 
lies,  is  the  weak  point  of  the  armature,  the  one  that  gives  the  greatest 
amount  of  trouble  in  maintenance,  and  therefore  it  is  the  one  over 
which  most  care  is  taken.  If  the  insulation  is  not  properly  carried 
out,  if  at  any  point  a  minute  pin  point  of  iron  has  been  left  in  the 
slot,  even  if  some  non-metallic  dust  has  been  left,  the  vibration  of 
the  machine  may  gradually  cause  the  lessening  of  the  thickness  of 
the  insulation  between  the  copper  conductors  and  the  iron,  with  the 
result  that  sparking  may  occur  at  that  point,  when  some  heavy  load 
is  taken  off  the  service,  induction  being  particularly  heavy  on  those 
occasions,  and  pressures  many  times  greater  than  that  of  the  service 
itself  being  often  present.  If  a  spark  does  pass,  the  machine  is 
wrecked.  In  the  modern  machine  it  is  fair  to  say  that  breakdowns 
of  that  kind  are  comparatively  rare.  In  the  machine  of  twenty  years 
ago  they  were  only  too  frequent. 

The  cylindrical  polar  space  in  which  the  armature  revolves  may 
have  only  one  magnetic  field,  being  then  known  as  a  bi-polar,  or  two- 
pole  machine;  but  more  frequently  in  modern  continuous  current 
dynamos  there  are  two  or  more  magnetic  fields,  with  four  or  more 
magnet  poles,  and  it  is  then  known  as  a  multipolar  machine. 

The  two-pole  machine  has  almost  died  out.  It  was,  however,  the 
form  in  which  all  the  early  machines  were  constructed.  In  the  latest 
form  of  the  bi-polar  machine  the  electro-magnets  consist  of  two  slabs  of 
iron  or  special  magnet  steel,  rising  from  a  base  plate  with  which  they 
are  cast,  where  the  special  magnet  steel  is  employed,  the  base  plate 
being  long  enough  to  accommodate  the  pedestals  to  which  the  bearings 
are  fixed,  in  which  the  armature  axle  runs.  The  two  slabs  of  iron 
are  placed  at  sufficient  distance  apart  to  allow  of  the  spools  carrying 


1 72  ELECTRICITY  IN   MINING 

the  magnetizing  coils  being  slipped  over  them,  and  their  upper  ends 
are  bored  out  to  form  the  polar  space  mentioned.  In  the  Parker  two- 
pole  machine,  which  is  the  latest  survival  of  the  type,  the  pole  pieces 
are  hinged  on  a  horizontal  line  at  about  the  middle  of  the  diameter  of 
the  space  occupied  by  the  armature,  so  that  the  upper  portions  of  the 
pole  pieces  can  be  thrown  back,  and  the  armature  lifted  out  vertically, 
with  the  smallest  chance  of  damage. 

In  the  multipolar  dynamo,  the  bearings  for  the  armature  axle  are 
carried,  almost  invariably,  on  pedestals  rising  from  the  bedplate  to 
which  the  enclosing  field  magnet  ring  is  secured ;  and  it  is  arranged, 
in  some  of  the  larger  sizes,  to  run  the  enclosing  ring  back,  clear  of 
the  armature,  upon  an  extension  of  the  bedplate,  the  armature  coils 
then  being  easily  got  at  for  repairs. 

The  Winding  of  Continuous  Current  Armature  Coils. — There 
have  been  two  forms  of  winding  of  the  coils  of  continuous  current 
armatures,  known  respectively  as  ring  winding,  the  armature  being 
known  as  the  ring  armature ;  and  drum  winding,  the  armature  being 
known  as  the  drum  armature.  The  two  forms  of  armature  and  the 
two  windings  are  taken  from  the  two  early  machines,  the  Gramme 
and  the  Siemens.  The  ring  armature  has  practically  died  out.  Its 
construction  was  as  follows.  In  the  very  early  machines  a  ring 
of  iron  wire  was  formed  by  winding  purest  charcoal  wire  on  a 
former,  the  ring  forming  a  hollow  cylinder,  which  was  wrapped  with 
calico.  Cotton-covered  copper  wire  was  then  wound  transversely 
across  the  outside  and  through  the  inside  of  the  ring,  the  whole  being 
held  on  a  wooden  hub  driven  into  the  space  left  inside  the  copper 
wires  on  the  inside  of  the  ring,  the  commutator,  which  was  built  up 
very  much  as  in  the  modern  dynamo,  though  not  as  well  insulated, 
was  soldered  to  the  ends  of  the  wires,  and  the  axle  of  the  machine  was 
slipped  through  a  hole  and  keyway  in  the  wooden  hub,  and  through 
the  centre  of  the  insulating  rings  of  the  commutator,  the  whole  being 
tightened  up  by  a  couple  of  iron  nuts  pressing  against  the  end  of  the 
commutator,  the  back  end  of  the  armature  hub  butting  against  a 
flange  provided  for  it  on  the  axle.  Later,  the  iron  wire  ring  gave 
place  to  a  ring  built  up  of  thin  iron  plates,  with  paper  between,  or 
insulated  by  varnish,  the  plates  being  held  on  a  brass  spider  which 
was  keyed  on  the  driving  axle,  the  wires  being  wound  as  before  over 
the  outside  of  the  iron,  which  had  been  insulated  in  various  ways, 
and  through  the  space  left  on  the  inside  between  the  spider  arms  and 
the  core.  The  drum  armature  consisted  originally  of  a  long  cylinder 
of  iron  wire  wound  very  much  in  the  same  manner  as  the  Gramme 
core,  but  from  three  to  four  times  as  long,  and  from  the  first  the 
copper  wires  were  only  wound  upon  the  outside  of  the  iron  core,  this 
having  been  insulated  in  a  similar  manner  to  the  core  of  the  Gramme 
ring,  and  being  held  on  a  wooden  hub  driven  into  the  middle.  In 


THE   GENERATION   OF  ELECTRICITY  173 

the  drum  armature  the  coils  of  the  copper  wires  crossed  each  other, 
both  at  the  commutator  end  and  at  the  back  of  the  armature,  and  in 
the  early  forms  there  were  always  two  layers  of  wire.  As  each  coil 
occupied  a  certain  portion  of  the  circumference  at  opposite  ends  of  a 
diameter,  when  a  certain  number  of  coils  had  been  wound,  the  arma- 
ture was  completely  covered,  but  there  were  only  half  the  coils  on 
that  were  required,  so  a  second  layer  was  put  on,  commencing  at  the 
opposite  side  of  the  armature  to  that  at  which  the  first  layer  com- 
menced, and  a  second  lot  of  coils  were  wound  over  the  first,  insulation 
being  placed  between  the  two  layers  and  between  the  wires,  where 
they  crossed  at  the  back  and  in  front  of  the  armature.  It  was  neces- 
sary in  this  form  of  winding  to  allow  a  large  space,  both  at  the  back 
and  the  front,  so  that  the  ends  of  the  coils  which  came  out  to  the 
commutator  were  very  much  longer  than  those  in  the  Gramme  ring. 
In  both  the  Gramme  ring  and  the  early  Siemens'  machine  successive 
coils  were  connected  together  in  series,  the  end  of  No.  1  coil  being 
connected  to  the  commencement  of  No.  2,  the  end  of  No.  2  to  the 
commencement  of  No.  3,  and  so  on.  In  the  Siemens'  armature  with 
two  layers,  the  under  layer  formed  the  coils  connected  to  one  half  of 
the  commutator,  while  the  layer  on  top  formed  the  coils  connected  to 
the  opposite  half  of  the  commutator.  This  construction  has  also 
disappeared.  There  were  frequent  troubles  with  that  form  of  winding 
from  the  breakdown  of  the  insulation  between  the  wires  which  crossed 
each  other  at  the  back  and  front  of  the  armature.  Wires  between 
which  a  large  portion  of  the  total  pressure  generated  by  the  machines 
existed  were  often  very  close  together,  and  the  insulation  would 
gradually  break  down,  sparking  between  the  wires  resulting,  and  coils, 
or  portions  of  them,  burning.  In  both  the  early  Siemens'  and  the 
Gramme  ring  armatures  there  were  also  troubles  only  too  frequently, 
from  the  breakdown  of  the  insulation  between  the  coils  and  the  iron 
core,  this  leading,  as  will  be  explained  in  Chapter  VII.,  to  burning  out 
of  some  of  the  armature  coils.  The  next  step  in  the  drum  armature  was, 
only  one  layer  was  wound,  and  each  alternate  coil  was  connected  to 
the  opposite  brush.  There  were  only  two  sets  of  brushes  to  the  early 
machines.  This  arrangement  meant  that  the  full  pressure  generated 
by  the  machine  existed  between  the  adjacent  coils,  and  this,  again,  in 
the  early  days  of  generator  construction,  led  to  sparking  between 
adjacent  coils  and  to  the  burning  of  portions  of  the  armature. 
Modern  practice  has  settled  down  to  one  form  of  armature,  the  drum, 
to  one  construction  of  core,  that  which  has  been  described,  the  slotted; 
but  there  are  two  forms  of  windings,  known  as  the  wave  and  lap  wind- 
ing, for  the  arrangement  of  which  the  reader  is  referred  to  the  text- 
books specially  devoted  to  the  subject.  In  both  forms  it  is  arranged 
that  a  certain  number  of  coils  are  always  delivering  a  positive  current 
to  the  positive  brushes,  and  certain  other  coils  are  at  the  same  time 


'74 


ELECTRICITY  IN   MINING 


receiving  an  equal  negative  current  through  the  negative  brushes. 
In  all  continuous  current  machines  the  coils  on  the  armature  form  one 
continuous  loop,  as  if  wound  from  one  length  of  wire,  the  ends  of 
certain  coils  being  connected  together  to  form  the  continuous  ring, 
and  these  junctions  connected  to  segments  of  the  commutator. 


The  Commutator 

The  commutator  is,  perhaps,  the  most  important  point  of  the  con- 
tinuous current  machine.  It  is  certainly  the  one  which  gives  the 
greatest  amount  of  trouble.  It  is  a  hollow  cylinder  built  up  of  a 
number  of  segments  of  copper,  separated  from  each  other  by  plates  of 
mica,  the  whole  being  held  together  by  rings  of  micanite,  held  by  iron 
rings  upon  a  boss  carried  by  the  driving  axle  of  the  armature.  The 
copper  segments  are  made  from  hard-drawn  pure  copper.  Purity  of 
the  copper  is  of  the  highest  importance.  In  some  machines  the  copper 
segments  are  cast  or  drop  forged,  but  are  always  of  the  very  purest 
copper.  There  is  a  difficulty  in  casting  copper  pure  and  hard,  but  it 
is  an  advantage  to  have  the  segments  cast  in  the  form  in  which  they 
are  to  be  assembled  in  the  commutator.  One  of  the  difficulties  in  the 
construction  of  a  commutator,  especially  those  of  the  large  multipolar 
machines  now  in  use,  is  the  holding  the  commutator  together  after  it 
is  built  up,  in  such  a  manner  that  it  will  withstand  the  twisting 
strains  brought  against  it  as  the  armature  revolves.  To  meet  this 
difficulty  the  segments  are  recessed  in  the  lower  portions  in  various 
forms,  the  recesses,  when  the  segments  are  built  into  a  cylinder,  form- 
ing the  channels  in  which  micanite  rings  are  fixed.  The  difficulty 
of  the  problem  is,  giving  the  whole  structure  sufficient  mechanical 
strength,  while  maintaining  the  perfect  insulation  of  each  individual 
segment  from  its  neighbour,  and  of  the  whole  of  them  from  the  axle 
or  the  boss  upon  which  they  are  built  up.  In  early  machines  wood 
rings  were  employed  for  insulation,  held  in  channels  recessed  in  the 
ends  of  the  segments.  The  wood  rings  frequently  split,  and  the  coil 
nearest  the  split  then  burned.  Later,  vulcanized  fibre  and  vulcanite 
were  employed,  but  these  also  were  not  satisfactory.  Vulcanite  is 
very  liable  to  split,  and  vulcanized  fibre  did  not  give  good  mechanical 
strength,  in  the  form  of  the  rings  that  were  turned  for  the  purpose. 
Modern  practice  has  adopted  the  substance  known  as  micanite. 
Mica  is  a  very  peculiar  substance.  It  has  a  very  high  insulation 
resistance,  and  it  also,  which  is  more  important,  resists  sparking 
through  it  very  much  better  than  almost  any  known  substance.  But 
it  exists  only  in  plates  made  up  of  very  thin  laminae.  You  can  have 
a  mica  plate  as  thin  or  as  thick  as  you  like,  but  its  two  sides  will 
always  be.  parallel,  and  it  will  split  longitudinally  as  much  as  you 


THE   GENERATION    OF  ELECTRICITY 


175 


please.  In  micanite  the  mica  is  reduced  to  its  chips,  the  chips  being 
in  the  form  of  very  small  laminae,  and  these  are  made  into  a  sort  of 
paste  by  the  aid  of  one  of  the  insulating  varnishes  that  have  been 
introduced  during  recent  years,  which  withstand  moisture  and  heat, 
and  the  whole  is  moulded  into  the  form  of  rings,  formed  under 
hydraulic  pressure.  By  this  means  strong  rings  have  been  pro- 
duced, having  very  high  insulating  qualities,  and  comparatively  great 
mechanical  strength,  the 
result  being  that  one  of  the 
serious  troubles  in  con- 
nection with  commutator 
building  has  been  practi- 
cally got  rid  of.  Each  com- 
mutator segment  has  an  arm 
attached  to  it,  standing 
radially  out  from  it,  and  to 
this  arm  the  ends  of  the 
coils  of  the  armature  form- 
ing the  junctions  mentioned 
above  are  secured.  The  arm 
is  sometimes  cast  with  the 
commutator  segment,  but 
is  more  frequently  secured 
to  it  by  screws  and  solder. 
The  armature  wires  are 
secured  to  the  commu- 
tator arms  in  the  smaller 

machines  by  soldering,  the  FIG.  81.— Showing  a  Commutator  built  up  ready 
arms  being  well  tinned,  and       *<*  the  Insulating  Kings,  the  jiecess  for  one  of 

the  ends  of  the  wires  being 
also  well  tinned.  With 
large  machines  the  wires  are  held  in  crutches  formed  in  the  commu- 
tator arms  by  screws,  and  the  whole  mass  is  also  sweated  up  together 
with  solder.  Holding  the  ends  of  the  pairs  of  wires  firmly  connected 
to  their  proper  commutator  segment  and  to  each  other  is  another 
of  the  troublesome  problems  of  dynamo  construction.  It  is  referred 
to  again  in  Chapter  VII.  Fig.  81  shows  a  commutator  built  up, 
ready  for  its  insulating  rings. 


which  is  shown  in  Front.    The  Lugs  for  the 
Armature  Wires  are  seen  at  the  Back. 


The  Excitation  of  Continuous  Current  Machines 

The  continuous  current  machine  is  self-exciting,  or  it  may  be 
excited%by  the  current  from  another  machine,  as  convenient.  As 
explained  in  Chapter  I.,  all  iron  that  has  once  been  subjected  to  a 


176 


ELECTRICITY   IN    MINING 


magnetizing  electric  current,  unless  the  magnetizing  current  only 
produced  magnetism  on  the  unstable  portion  of  the  magnetizing 
curve,  retains  a  small  quantity  of  magnetism,  after  the  magnetizing 
current  has  ceased,  and  this  small  amount  of  magnetism  is  sufficient 
to  create  a  small  current  in  the  armature  coil  when  the  machine  is 
run.  This  small  current,  being  passed  through  the  coils  of  the  field 
magnets,  increases  the  magnetism  created  in  them  slightly,  the  in- 
creased magnetism  giving  rise  to  increased  current  in  the  armature 
coils,  this  again  increasing  the  magnetism,  and  so  on,  until  the  full 


— /ARMATURE. 


FIG.  82. — Diagram  of  Connections  of  Separately  Excited  Generator  with 
Adjustable  Bheostat  in  the  Circuit  of  the  Field  Coils.  +  and  — 
are  the  Positions  of  the  Brushes. 

magnetic  field  is  created,  and  the  full  pressure  is  generated,  for  which 
the  machine  is  designed.  This  is  the  action  which  takes  place.  In 
practice  it  occupies  only  a  very  short  interval  of  time,  and  its 
existence  is  only  known  when  from  any  cause  a  machine  fails  to 
"build  up,"  as  it  is  termed.  In  large  generating  stations  it  is 
frequently  arranged,  even  where  continuous  current  machines  are 
employed,  to  run  a  separate  generator  for  the  current  required  by 
the  field  magnets.  The  connections  for  this  are  shown,  for  a  single 
machine,  in  Fig.  82.  Continuous  current  generators  may,  however,  be 
self-excited  on  either  the  series,  shunt,  or  compound  arrangement. 


THE   GENERATION   OF   ELECTRICITY  177 

In  series-wound  machines  the  coils  of  the  field  magnet  are  wound 
with  thick  wire,  sufficiently  large  to  take  the  whole  current  gene- 
rated by  the  machine.  In  the  case  of  multipolar  machines,  it  may 
be  arranged  that  the  current  is  split  up  between  the  pairs  of  field 
magnets  in  parallel,  or  it  may  go  round  the  whole  of  them  in  series. 
Fig.  83  is  a  diagram  of  the  connections  of  a  series- wound  machine. 
Very  few  series-wound  generators  are  now  made,  because  the  shunt- 
wound  and  the  compound-wound  answer  all  purposes  very  much 
better.  The  series-wound  machine  reflects  every  change  in  the 
external  resistance  of  the  circuit,  in  a  sense  which  is  against  the 
efficient  working  of  the  apparatus.  Thus,  supposing  a  machine  to 
be  running  at  a  certain  speed,  furnishing  a  certain  current,  with  a 
certain  pressure  between  its  terminals.  If  the  resistance  of  the  outer 
circuit  through  which  the  current  is  passing  increases,  the  current 
passing  through  the  whole  circuit,  including  the  coils  of  the  field 


FIG.  83.— Diagram  of  Series-wound  Generator.  One  End  of  the  Field  Coils 
is  connected  to  one  Brush,  usually  the  Negative,  the  other  Brush  and 
the  other  End  of  the  Field  Coils  forming  the  Terminals. 

magnet  and  the  armature,  is  reduced,  and  this  means  that  the  load 
upon  the  driving  engine,  whatever  it  may  be,  is  also  reduced,  with 
the  result  that  usually,  unless  the  engine  is  exceedingly  well 
governed,  it  increases  its  speed,  and  a  current  is  produced  in  the 
outer  circuit,  that  is  not  required.  On  the  other  hand,  if  the  resist- 
ance of  the  outer  circuit  decreases,  the  increased  current  passing 
brings  an  additional  load  upon  the  engine,  which  tends  to  slow  up. 
The  most  striking  instance  of  this  is  the  case  of  one  or  two  arc  lamps 
worked  by  current  from  a  series  machine.  If  the  lamps  burn  long 
arcs,  the  engine  will  increase  its  speed,  and  if  one  of  them,  as 
nearly  always  happens,  goes  out,  its  carbons  coming  into  contact,  the 

N 


i78 


ELECTRICITY   IN  MINING 


engine  is  pulled  up.  The  series-wound  generator  is  suitable  for 
running  a  number  of  arcs  in  series,  and  it  is  employed  in  America 
for  this  purpose,  but  in  the  special  form  well  known  in  this  country 
some  years  ago,  of  the  Brush  arc-lighting  machine.  In  America 
they  run  as  many  as  3  30  lamps  from  a  single  Brush  machine  in  two 
sets  of  65  lamps  each.  For  mining  work,  however,  it  is  much  more 
satisfactory,  and  generally  much  more  convenient,  to  take  current 
from  the  service,  by  one  of  the  methods  that  have  been  described  in 
Chapter  III.  If  any  mine  manager,  however,  has  a  series-wound 
machine,  and  wishes  to  run  some  arc  lamps  from  it,  he  can  do  so, 
providing  that  he  arranges  cutouts  to  his  lamps  in  case  they  go  out. 
Perhaps  the  most  important  part  of  the  Brush  arc-lighting  system 
was  the  arrangement  by  which  the  pressure  was  reduced  in  case  a 
lamp  went  out,  or  whenever  the  resistance  of  the  circuit  was 
decreased.  The  series-wound  generator  may  also  be  used  for  furnish- 
ing current  to  drive  a  series- wound  motor  where  it  is  convenient  for 
other  reasons ;  but  again,  it  will  be  far  more  satisfactory  to  take 
current  from  the  supply  service. 

The  Shunt-wound  Generator. — In  the  shunt-wound  generator 
the  field  magnet  coils  are  energized  by  only  a  small  portion  of  the 


_AAAAAAAAAAA/v_ 

FIELD    Coius 


FIG.  84. — Diagram  of  the  Connections  of  the  Shunt-wound  Generator. 
+  and  -  are  the-  Brushes.  The  Ends  of  the  Field  Coils  are 
connected  to  the  Brushes. 

current  generated  by  the  armatures.  The  field  magnet  coils  are 
wound  with  fine  wire,  to  a  resistance  such  that  only  a  small  fraction 
of  the  armature  current  passes  through  them,  and  the  ends  of  the 
field  magnet  coils  are  connected  to  the  brushes,  which  form  the 


THE    GENERATION   OF  ELECTRICITY 


179 


v_y 


terminals  of  the  machine,  and  to  which  the  cables  from  the  outer 

circuit  are   also   connected,  or  to  terminals  fixed  on  the  machine, 

having  leads  connecting   them  to  the  brushes.     Fig.  84  shows   the 

arrangement  of  the  connections  of  the  shunt  machine.     When  no 

current  is  taken   by  the  external  circuit,  the  only  current  passing 

through    the    armature  of   the   shunt-wound    generator  when    the 

machine  is  running,  and  is  fully  excited,  is  the  small  current  passing 

through  the  field  coils,  so  that  the  pressure  between  the  brushes  is 

very  nearly  equal  to  the  total  _^^ 

pressure  generated  by  the  arma-  S*^        ^S. 

ture.     It  will  be  remembered  /  \ 

that   every  resistance  makes  a 

charge  upon  the   pressure  de- 

livered to  it,  for  the  passage  of 

a  current  through  it,  the  charge 

being  measured  by  the  formula 

E  =  CK,  where  E  is  the  charge 

upon   the    pressure,   C   is   the 

current  passing,  and  K  is  the 

resistance.    As  the  current  pass- 

ing into  the   outer  circuit  in- 

creases, and  therefore  the  current 

through  the  armature  also  in- 

creases, the  charge  made  upon 

the    pressure    created    by    the 

armature   coils    increases,   and 

the  pressure  between  the  brushes 

decreases.     This  causes  the  cur- 

rent passing  in  the  field  coils 

to  decrease,  the  strength  of  the 

magnetic    field    in   which    the 


SHUNT  COILS 


RHCOSTAT 


armature  coils  are  moving  to  FIG.  85.-Diagram  of  Shunt-wound  Gene- 
,1  rator    with  Adjustable  Kheostat   in    the 


Adjustable 

Circuit  of  the  Field  Coils.  The  Arrange- 
ment enables  the  Pressure  to  be  kept 
Constant,  with  Constant  Speed  and  Vary- 
ing Current  in  the  External  Circuit,  or 
both  to  be  Varied  at  Will. 


decrease,  and  reduces  the  pres- 
sure created  by  the  armature 
coils,  this  reducing  the  pressure 
at  the  brushes,  and  still  further 
reducing  the  strength  of  the 
current  in  the  field  coils,  and  so  on.  If  the  external  resistance 
is  steadily  decreased  so  that  more  and  more  current  passes  in 
the  outer  circuit,  the  pressure  generated  by  the  machine  is  also 
steadily  decreased,  until  a  critical  point  is  reached,  when,  if  an 
attempt  is  made  to  pass  more  current  into  the  outer  circuit  by  still 
further  lowering  the  resistance,  both  pressure  and  current  fall,  and 
when  the  terminals  of  the  machine  are  short-circuited,  when 
there  is  no  resistance  between  them,  the  machine  generates  no 


i8o 


ELECTRICITY   IN   MINING 


pressure  and  no  current.  It  will  be  understood  that  increased 
current  is  taken  from  any  machine  by  lowering  the  resistance 
of  the  external  circuit  —  say  by  switching  on  a  larger  number  of 
parallel  circuits,  such  as  lamps  or  motors.  It  will  be  seen  also 
that  the  variation  of  the  pressure  at  the  terminals  of  the  shunt- 
wound  motor  is  inversely  as  the  resistance  of  the  armature  coils.  If 
it  were  possible  to  build  the  armature  of  a  shunt-  wound  generator 
with  no  resistance,  the  pressure  at  its  terminals  would  be  constant, 
and  the  lower  the  resistance  of  the  armature  coils,  the  smaller  is  the 
variation  of  the  pressure  at  its  terminals,  because  the  charge  upon 
the  pressure  generated  is  smaller.  In  all  shunt-wound  generators 
there  is  a  certain  range  of  current,  over  which  the  variation  in  pressure 
is  small.  Electrical  engineers  express  the  fact  by  saying  that  the 
machine  has  a  flat  curve  up  to  a  certain  point.  Fig.  85  shows  the 
connections,  with  a  shunt-  wound  generator,  for  regulating  the  current 
in  the  field  coils  by  an  adjustable  resistance,  enabling  the  speed  to  be 
maintained  constant. 

The    Compound-wound    Generator.—  The    compound-wound 
machine  is  a  combination   of  the   shunt-wound    and   series-wound 


COIL.S 


AAAAAAAAAAAA 


5HUNT   COILS 


FIG.  86. — Diagram  of  the  Conriections,of  a  Compound- wound  Generator. 
The  Ends  of  the  Shunt  Coils  are  connected  to  the  Brushes,  and  one 
End  of  the  Series  Coil  to  one  Brush,  usually  the  Negative. 

machines,  or  as  the  author  prefers  to  express  it,  it  is  the  shunt- 
wound  machine  with  a  few  turns  of  series  winding  on  its  field 
magnets,  sufficient  to  make  up  the  loss  in  pressure  due  to  the  charge 
made  by  the  resistance  of  the  armature  for  the  passage  of  the  current 


THE   GENERATION   OF   ELECTRICITY  181 

through  it.  The  series  coils  create  electro-magnetism  in  the  same 
manner  as  the  shunt  coils,  and  as  if  they  were  independently  exciting 
the  machine.  That  is  to  say,  they  increase  the  strength  of  the 
magnetic  field,  thereby  increasing  the  pressure  generated  by  the 
armature  coils,  and  they  must  be  of  sufficient  number  to  create 
sufficient  increased  magnetism  to  provide  the  additional  pressure 
required  to  make  good  the  charge  for  the  armature  coils,  and  also  the 
charge  for  passing  through  the  series  coils  themselves,  the  current 
from  the  external  circuit  being  taken  from  one  brush  and  the  end 
of  the  series  coils,  as  shown  in  Fig.  86.  The  characteristic  curve  of 
the  compound-wound  continuous  current  generator  is  a  straight  line 
within  the  limits  of  the  machine.  The  machine,  however,  may  be 
constructed,  if  desired,  and  often  is,  to  "  compound  up,"  as  it  is 
termed.  The  series  coils  are  made  a  little  longer  than  is  necessary 
to  give  the  additional  pressure  for  the  charge  made  by  the  armature 
and  field  coils  themselves,  and  this  provides  a  pressure  which  slightly 
increases,  in  any  ratio  that  may  be  desired,  as  the  current  taken  from 
the  machine  increases.  It  is  used  for  delivering  a  constant  pressure 
with  varying  current  at  any  point  selected  at  a  distance  from  the 
machine.  Thus  the  pressure  at  the  pit  bottom,  or  at  a  distributing 
point  in-bye,  may  be  constant.  It  must  be  understood,  however,  that 
when  the  machine  is  arranged  to  "  compound  up,"  the  pressure  at 
its  terminals,  and  at  any  points  between  its  terminals  and  the  point 
of  constant  pressure,  varies  with  the  current,  so  that  if  a  supply  of 
current  for  lamps  is  taken  from  the  terminals  of  the  machine 
with  this  arrangement,  the  lamps  will  be  subject  to  a  varying,  and 
sometimes  dangerous  pressure,  unless  some  provision  is  made  to 
neutralize  the  increased  pressure  as  the  current  increases.  This,  how- 
ever, is  easily  done  if  it  is  worth  while  for  other  reasons.  A  switch 
may  be  inserted  in  the  circuit,  with  an  adjustable  resistance  which  is 
thrown  in  or  taken  out  as  the  current  increases  or  falls  in  the  main 
circuit,  and  the  switch  may  be  worked  by  a  solenoid  whose  coils  are 
connected  in  the  main  circuit,  or  a  branch  of  it  that  varies  in  the 
same  proportion. 

Brushes  and  Brush  Gear  and  Connections  of  Armature 
Coils. — In  the  two-pole  machine  the  whole  of  the  connections  are 
very  simple.  There  are  only  two  sets  of  brushes  placed  at  opposite 
ends  of  a  diameter  of  the  commutator,  and  the  current  is  taken 
directly  from  the  brushes  to  the  field  coils,  or  otherwise.  With 
multipolar  machines,  however,  a  different  construction  is  necessary. 
With  four-pole  machines,  current  is  being  delivered  in  two  quadrants 
of  the  armature  in  the  same  direction,  and  in  the  other  two  quadrants 
in  the  opposite  direction,  and  it  is  therefore  necessary  to  collect  the 
current  at  four  points  instead  of  two.  With  six-pole  machines  it  must 
be  collected  at  six  points,  and  with  eight-pole  machines  at  eight 


1 82  ELECTRICITY  IN   MINING 

points.  In  the  early  multipolar  machines  the  coils  that  were 
generating  current  in  the  same  direction  at  the  same  time,  were 
connected  together  by  wires  carried  round  between  the  commutator 
and  the  front  of  the  armature,  and  only  two  sets  of  brushes  were 
employed,  these  being  fixed  in  the  four-pole  machines  90°  apart.  In 
the  modern  dynamo,  however,  there  are  brushes  for  each  point  of 
collection,  the  brushes  being  either  carbon  blocks  or  copper,  made 
up  as  will  be  described,  in  either  case  held  in  shoes  of  various  forms, 
the  shoes  being  fixed  mechanically  to  spindles  parallel  with  the 
commutator,  and  held  by,  but  insulated  from,  usually  a  massive  iron 
ring,  supported  on  the  bearing  at  the  armature  end,  or  by  brackets 
on  the  enclosing  ring  of  the  field  magnets,  as  shown  in  Plate  7 A.  The 
current  from  the  commutator  is  delivered  to  the  brushes,  from  them 
to  the  spindles  which  hold  them,  and  these  spindles  are  connected,  by 
conductors  of  sufficient  size  to  eliminate  resistance,  with  the  spindles 
of  the  other  brushes  which  are  delivering  current  in  the  same  direction. 
Practically  a  multipolar  machine  consists  of  a  number  of  machines, 
held  together  by  the  enclosing  ring,  each  machine  consisting  of  two 
adjacent  magnet  cores  and  their  windings,  the  piece  of  the  enclosing 
magnet  cylinder  behind  them,  and  the  piece  of  the  armature  core  in 
front  of  them,  and  the  current  generated  by  each  machine  is  delivered 
to  its  own  set  of  brushes,  and  thence  to  one  set  of  terminals  by  the 
brushes  being  connected  in  parallel.  In  addition,  in  many  forms  of 
continuous  current  machines  now  made,  there  are  equalizing  con- 
ductors, connecting  points  on  the  commutator  together,  at  which  the 
pressure  should  be  the  same  at  each  instant,  the  idea  being  to 
equalize  the  generation  of  current  all  round  the  machine.  The  current 
from  the  different  brush  holders  are  brought  to  massive  terminals 
fixed  on  insulating  blocks  upon  any  convenient  part  of  the  machine. 

Carbon  and  Copper  Brushes. — Carbon  brushes,  as  they  are 
termed,  though  they  are  blocks  of  carbon,  are  employed  very  much 
more  frequently  in  modern  machines  than  copper  brushes.  The 
name  arises  from  the  fact  that  the  early  arrangement  for  collecting 
the  current  from  the  commutator  was  a  brush,  made  of  a  number  of 
copper  wires  soldered  together  at  one  end,  and  held  so  that  the  loose 
ends  bore  upon  the  commutator.  Copper  brushes  are  still  employed, 
and  are  very  much  preferred  by  some  engineers,  but  the  simple 
form  described  above  has  been  very  much  departed  from.  One 
form  of  copper  brush  made  by  the  Wirt  Co.  consists  of  leaves  of 
thin  copper,  and  of  a  comparatively  high  resistance  metal,  placed 
alternately  one  above  the  other,  one  end  of  the  laminated  mass 
resting  on  the  commutator,  and  the  other  being  soldered  together. 
Carbon  brushes  are  of  various  forms  and  various  sections.  A 
favourite  form  is,  a  block  having  a  rectangular  section  where  it  meets 
the  commutator,  and  a  wedge-shaped  section  to  fit  into  a  slide  on 


THE   GENERATION   OF  ELECTRICITY  183 

the  end  of  a  substantial  brass  plate.  Carbon  brushes  are  sometimes 
coppered,  and  are  sometimes  used  without.  Where  carbon  brushes 
are  employed,  a  very  much  larger  surface  in  contact  with  the 
commutator  must  be  employed,  the  density  of  current  taken  by  any 
carbon  brush  not  exceeding  40  amperes  per  square  inch,  while  with 
copper  brushes  the  density  may  be  as  large  as  200  amperes  per 
square  inch.  The  undoubtedly  better  behaviour  of  carbon  brushes 
over  the  ordinary  copper  brush,  is  the  source  of  some  controversy 
among  electrical  engineers.  The  office  of  the  brush  in  a  continuous 
current  machine  is  twofold.  It  has  to  collect  the  current  generated 
in  a  half,  quarter,  or  other  portion  of  the  armature  coils,  and  pass 
it  on  to  the  outer  circuit.  It  also  has  to  accept  the  reversal  of  the 
current  in  the  coil  which  is  passing  under  it.  Taking  any  section  of 
the  armature  whose  coils  are  passing  up  towards  the  brush,  all  the 
coils  are  generating  current,  which  is  being  poured  through  the  coils 
in  front,  to  that  which  is  under  the  brush,  and  thence  to  the  outer 
circuit.  When  an  individual  coil  arrives  at  the  brush,  it  first  acts 
as  the  connection  between  the  coils  behind  it  and  the  brush,  the 
current  passing  from  it  to  its  section  of  the  commutator,  and  then  it 
is  itself  short-circuited  for  a  very  minute  interval,  while  it  is  passing 
under  the  brush.  Then  the  current  passing  in  it  is  reversed,  and 
the  next  instant  the  connection  between  it  and  the  brush  is  broken, 
and  it  is  at  this  instant  that  sparking  occurs.  While  the  coil  is 
short-circuited,  during  the  period  that  the  adjacent  segments  of  the 
commutator  to  which  it  is  connected  are  passing  under  the  brush,  a 
very  heavy  current  is  induced  in  the  coil,  and  it  is  this  current  which 
is  broken,  and  which  causes  the  sparking  when  the  coil  passes  from 
under  the  brush.  One  effect  appears  to  be  undoubted,  carbon  brushes 
wear  the  surface  of  the  commutator  less  than  copper,  they  create 
less  friction,  carbon  itself  in  its  best  forms  having  a  considerable 
lubricating  value.  In  addition,  the  spark  which  is  formed  converts 
the  carbon  into  vapour,  which  is  carried  off  by  the  revolution  of  the 
armature,  and  does  less  harm  in  that  way  than  the  equivalent  action 
in  the  case  of  the  copper  brush.  Similar  difference  of  opinion  exists 
also  as  to  the  advisability  of  coppering  carbon  brushes.  The  copper 
coating  reduces  the  resistance  of  the  carbon,  this  being  an  advantage 
in  some  respects,  and  not  in  others.  If,  however,  the  copper  coating 
is  not  carefully  put  on,  loose  laminae  of  copper  are  apt  to  be  left  in 
the  neighbourhood  of  the  brush,  and  to  give  trouble.  It  is  thought 
that  the  higher  resistance  of  the  carbon  brush  accounts  for  the 
lessened  sparking  when  carbon  brushes  are  employed ;  but  in  the 
author's  opinion  this  explanation  is  hardly  tenable,  since  the  number 
of  carbon  brushes  must  be  increased  until  the  resistance  of  contact 
and  of  brush  is  the  same  as  with  copper  brushes. 

There  are  various  forms  of  brush  holders,  the  majority,^ where 


184 


ELECTRICITY   IN   MINING 


carbon  brushes  are  employed,  being  arranged  to  keep  the  carbon  block 
bearing  radially  on  the  surface  of  the  commutators.  With  copper 
the  more  favourite  arrangement  is  one  which  holds  the  brush  tan- 
gential to  the  surface.  In  the  latest  forms  of  brush  holders,  arrange- 
ments are  made  for  throwing  an  individual  brush  back,  clear  of  the 
commutator,  so  that  it  can  be  trimmed  without  danger ;  and  in  the 
great  majority  also,  arrangements  are  made  for  regulating  the  pressure 
with  which  the  brush  bears  upon  the  commutator,  while  the  machine 
is  running.  One  form  of  brush  holder,  with  a  carbon  brush,  is  shown 
in  Fig.  87.  Sparking  at  the  brushes  in  all  modern  machines  is  very 
small  indeed  under  all  conditions,  unless  there  are  very  large  changes 
of  load,  and  even  then  with  some  forms  of  machine  the  sparking  is 


FIG.  87. — One  Form  of  Brush  Holder  with  Carbon  Brush,  made  by  Messrs. 
Santoni.  The  Arrangement  of  the  Carbon  and  the  Method  of  Regulating 
the  Pressure  are  shown  very  clearly. 

still  very  small.  This  result  is  due  mainly  to  the  improvement  in 
the  general  construction  of  dynamo  machines,  and  is  applicable  to 
generators  and  motors.  The  conditions  for  the  smallest  amount  of 
sparking  are  well  known.  They  are,  in  the  first  place,  that  the 
armature  coils  shall  be  divided  up  as  much  as  possible,  so  that  each 
individual  coil  has  only  a  very  few  turns,  and  therefore  its  self- 
induction  is  very  small ;  and  secondly,  that  the  field  created  by  the 
field  magnets  shall  be  always  able  to  overpower  that  created  by  the 
armature,  and  particularly  at  the  point  of  commutation.  It  should, 
perhaps,  be  explained  that  some  of  the  trouble  from  sparking  is  due 


PLATE  9A.— A  Sub-station  fitted  with  Westinghouse  Rotary  Converters.  The 
Station,  it  will  be  seen,  is  very  similar  to  a  Generating  Station,  but  there  are 
no  Driving  Engines. 


PLATE  9B.— Water  Power  Electricity  Generating  Station,  of  the  North  Wales 
Power  Co.,  fitted  by  Messrs.  Bruce,  Peebles  &  Co. 

[To  face  p.  184. 


THE   GENERATION   OF   ELECTRICITY  185 

to  the  fact  that  the  current  passing  in  the  armature  coils  creates  its 
own  magnetic  field,  and  that  the  resultant  field  in  which  the  armature 
coils  run  is  more  or  less  distorted  in  consequence. 

Continuous  Current  Machines  with  Commutating  or 
Auxiliary  Poles. — In  addition  to  the  improvements  in  construc- 
tion that  have  been  mentioned,  a  late  development  has  been  made, 
principally  for  use  with  motors,  but  also  applicable  to  continuous 
current  generators,  which  still  further  reduces  the  sparking  at  the 
brushes.  The  arrangement  consists  in  the  provision  of  auxiliary 
electro-magnets,  fixed  between  the  proper  field  magnets,  and  at  the 
points  of  commutation.  These  electro-magnets  come  into  action  at 
the  moment  when  the  self-induction  which  has  been  referred  to  is 
taking  place,  in  the  armature  coil  passing  under  the  brush,  and  are 
arranged  to  create  a  pressure  in  the  coil  under  commutation,  opposite 
to,  and  equal  to  that  created  by  the  self-induction  of  the  coil  in  the 
ordinary  field  of  the  machine,  thus  reducing  the  current  that  is 
broken  when  the  commutator  segment  passes  from  under  the  brush, 
to  very  small  proportions. 


The  Alternating  Current  Generator 

The  principal  difference  between  alternating  current  generators 
and  continuous  current  generators  is,  in  the  alternating  current 
generator  the  currents  are  allowed  to  be  delivered  into  the  outer 
circuit  exactly  as  they  are  generated.  In  the  continuous  current 
machine  each  coil  generates  currents  in  opposite  directions  at  different 
portions  of  the  revolution,  the  currents  being  arranged  all  in  one 
direction  by  the  commutator  and  the  brushes.  In  the  alternating 
current  machine  no  commutator  is  required,  the  only  arrangement 
for  delivering  the  current  from  the  armature  of  the  alternator  to  the 
outer  circuit  consisting  of,  where  the  armature  rotates,  brass  rings 
or  collars  carried  on  the  driving  axle,  but  insulated  from  it  and 
from  each  other  with  copper  brushes  bearing  upon  the  collars, 
and  taking  the  currents  from  them,  just  as  they  are  generated. 
There  is  never  any  break  between  the  brush  or  collecting  plate  and 
the  collar  it  bears  upon,  except  by  accident,  or  by  the  machine 
getting  out  of  order,  and  therefore  there  is  a  complete  absence  of  the 
sparking  which  is  such  a  noticeable  and  often  troublesome  feature  in 
connection  with  the  continuous  current  armature.  There  are  four 
forms  of  alternating  current  machines,  though  as  usual  modern 
practice  is  settling  down  to  one  form.  In  one  form,  which  is  perhaps 
the  oldest  arrangement  of  all,  the  armature  coils  are  carried  vertically 
on  the  edge  of  a  disc,  and  they  revolve  between  two  crowns  of  field 
magnets  also  arranged  in  vertical  planes.  In  this  form  of  machine 


1 86  ELECTRICITY   IN   MINING 

the  coils   are  wound  separately,  and  usually  of  a  sector  or  wedge 
shape,  the  conductors  being  in  the  larger  sizes  of  strip  copper  with  a 
strip  of  insulating  material  between,  the  coils  when  wound  being 
securely  fixed  to  a  disc  of  insulating  material,  which  in  its  turn 
is  securely  held  on  a  steel  disc  carried  by  the  revolving  axle.     The 
connections  of  the  adjacent  coils  of  the  armature  of  this  type  are 
arranged  in  the  reverse  direction,  the  reason  being,  the  field  magnet 
poles  are  arranged  round  the  machine  in  pairs,  and  so  that  north  and 
south  poles  alternate  with  each  other,  also  north  poles  in  one  crown 
of  field  magnets  face  south  poles  in  the  opposite  crown,  and  vice  versa. 
Each  double  pair  of  poles,  two  on  one  crown  and  two  on  the  other, 
form  practically  a  machine,  or  a  closed  magnetic  circuit,  the  lines  of 
force  passing  from  the  north  pole  of  one  to  the  south  pole  of  the  other, 
through  the  yoke  connecting  the  poles  on  that  crown  to  the  north 
pole  on  that  crown,  across  the  space  in  which  the  armature  revolves 
to  the  south  pole  of  the  first  crown,  and  through  the  yoke  to  the 
north  pole  again.     The  direction  of  the  lines  of  force,  it  will  be 
seen,  is  reversed  at  each  pair  of  poles.    Commencing  at  any  individual 
pair,  with  one  of  the  north  poles  of  the  crown,  say  on  the  left  of  the 
machine,  looking  at  the  side,  the  lines  of  force  will  pass  from  left 
to  right.     Between  the  next  pair  of  poles  they  will  pass  from  right 
to  left,  between  the  next  pair  from  left  to  right,  and  so  on.     Conse- 
quently, as  each  coil  comes  up  to  the  lines  of  force  passing  from  left 
to  right,  currents  are  generated  in  one  direction,  gradually  increasing 
as  the  coil  passes  into  the  field,  reaching  their  maximum  when  the 
coil  is  in  the  strongest  part  of  the  field,  and  gradually  decreasing  as 
it  passes  out  of  the  field.     While  this  is  going  on  with  one  coil,  the 
coil  which  is   approaching   the  next  pair   of    magnets   will  have 
pressures   and   currents  generated  in  it  in  the  opposite  direction, 
rising  and   falling  in   the   same  manner.      But  unless  some  corn- 
mutating   device  is   employed,  it  is  necessary  to  arrange  that  the 
currents  are  delivered  to  the  outer  circuit,  all  in  the  same  direction, 
all  rising  together,  and  all  falling  together,  and  the  pressures  created 
by  each  individual  coil  passing  through  each  set  of  lines  of  force, 
being   added   together  to  make   the   total  pressure  created  by  the 
machine.     This  is  accomplished  by  reversing  the  connections  of  the 
coils  as  explained.     The  arrangement  was  shown  very  beautifully  in 
the  early  Ferranti  armature,  in  which  the  copper  strip,  or  in  the  case 
of  small  machines,  copper  wire,  was  wound  around  pins  placed,  half 
of  them  at  the  edge  of  a  disc,  and  the  other  half  at  a  certain  distance 
radially  from  the  edge.     It  will  be  seen  that  alternate  turns  of  the 
winding  were  in  opposite  directions.     The  outer  portion  would  be 
passing  between  the  pairs  of  poles  in  which  the  lines  of  force  pass 
from  left  to  right,  while  the  inner  portion  was  passing  between  the 
next  pairs  of  poles  in  which  the  lines  of  force  were  from  right  to  left. 


THE   GENERATION   OF  ELECTRICITY  187 

The  pressures  would  be  in  opposite  directions  with  regard  to  the 
conductors  themselves,  but  would  be  in  one  direction  with  regard 
to  the  whole  of  the  winding  of  the  armature  and  the  outer  circuit. 
The  crowns  of  field  magnets  in  this  form  of  machine  are  held  in  two 
castings  standing  vertically  upon  a  bedplate,  and  so  arranged  that 
the  armature  axle  passes  through  the  centre,  the  bedplate  also 
carrying  the  pedestals  for  the  bearings  of  the  axle.  On  the  axle 
also  between  one  bearing  and  the  armature  are  the  collector  rings 
mentioned,  the  collector  brushes  being  held  by  fixed  brackets  secured 
to  the  rings  upon  which  the  field  magnets  are  fixed.  The  excitor 
dynamo  is  often  carried  in  this  type  of  machine,  as  in  others,  on  a 
casting  forming  a  part  of  the  bedplate  of  the  main  machine,  the 
field  magnets  of  the  excitor  machine,  which  is  nearly  always  bi-polar, 
being  secured  to  the  casting,  and  the  axle  of  the  armature  of  the 
excitor  running  in  a  small  bearing  carried  by  a  projection  of  the 
casting  upon  which  the  excitor  itself  is  fixed,  the  other  end  of 
the  excitor  axle  being  connected  mechanically  to  the  end  of  the 
axle  of  the  alternator.  This  arrangement  is  very  convenient  in  many 
respects,  and  is  very  compact.  It  lends  itself  to  regulation  of  the 
pressure  of  the  alternator,  because  if  the  speed  of  the  alternator  is 
increased,  the  speed  of  the  excitor  is  also  increased,  and  with  it, 
unless  the  current  is  reduced  at  the  rheostat,  as  will  be  explained, 
the  strength  of  the  exciting  current  of  the  field  magnets. 

In  another  form  of  alternator  which  was  made  by  several  firms 
at  one  time,  and  which  the  Westinghouse  Co.  have  adhered  to  for 
some  of  their  machines,  the  armature  is  very  similar  to  the  armature 
of  a  continuous  current  machine.  It  consists  of  thin  iron  plates, 
insulated  from  each  other,  with  slots  cut  in  their  periphery,  built  up 
into  a  drum,  the  slots  forming  longitudinal  grooves,  in  which  the 
coils  are  laid.  The  winding  of  the  coils  is  almost  identical  with 
that  of  the  winding  of  a  continuous  current  armature,  but  there  are 
no  breaks  or  junctions  in  the  wire,  and  no  commutator.  The  collecting 
rings  are  carried  on  the  axle  as  in  the  disc  machine,  the  copper 
brushes  bearing  on  them  being  carried  by  the  fixed  part  of  the 
machine,  usually  the  bearings  in  this  case;  but  the  rings  are  not 
connected  to  the  two  ends  of  the  armature  coils  as  in  the  disc 
machine,  they  are  connected  to  two  points  in  the  closed  winding 
180°  apart,  that  is  to  say,  at  opposite  ends  of  a  diameter  of  the 
armature.  In  this  type  of  machine,  also,  the  arrangement  of  the  field 
magnets  is  very  similar  to  that  of  the  multipolar  continuous  current 
dynamo.  There  is  the  same  enclosing  cylindrical  cylinder  of  iron  or 
mild  steel,  with  the  same  magnet  cores  projecting  radially  inwards, 
and  with  the  exciting  field  coils  held  on  the  magnet  cores.  The 
field  magnet  poles  are  arranged  around  the  cylinder  alternately  north 
and  south,  so  that  each  pair  of  poles,  with  the  portion  of  the  armature 


1 88  ELECTRICITY   IN   MINING 

between  them  and  the  portion  of  the  containing  cylindrical  yoke, 
may  be  looked  upon  as  a  complete  machine,  or  a  closed  magnetic 
circuit.  As  the  armature  revolves,  the  conductors  pass  across  the  lines 
of  force,  which,  as  in  the  disc  machine,  alternately  stretch  from  the 
magnet  pole  to  the  armature  core,  and  from  the  armature  core  to  the 
magnet  pole,  the  pressures  and  currents  created  being  opposite  as  each 
coil  passes  in  front  of  each  pole ;  but  the  arrangement  of  the  wind- 
ing, the  arrangement  of  the  coils  on  the  drum,  and  the  arrangement 
for  collecting  the  current,  perform  the  same  office  in  this  form  of 
alternator  as  reversing  the  connections  of  the  individual  coil  does 
in  the  disc  armature.  The  pressures  generated  in  the  individual 
coils  are  added  together  and  delivered  to  the  collector  rings.  The 
enclosing  field  magnet  ring  is  carried  on  a  bedplate,  which  also 
carries  the  pedestals  for  the  bearings  of  the  armature  axle,  and  in 
some  machines  of  this  type  the  excitor  dynamo  is  also  carried  on  an 
extension  of  the  bedplate,  its  axle  being  connected  mechanically  to 
the  axle  of  the  alternator ;  but  with  the  Westinghouse  machines  the 
arrangement  usually  is,  the  excitor  is  fixed  on  the  floor  in  the 
immediate  neighbourhood  of  the  alternator,  and  is  driven  by  ropes 
from  a  pulley  at  one  end  of  the  alternator  axle. 

In  another  form  of  alternator  the  field  magnets  are  arranged  to 
revolve,  the  armature  being  stationary,  and  this  form  is  gradually 
acquiring  favour,  as  it  presents  many  advantages.  There  are  no 
collecting  rings,  for  instance,  the  current  being  taken  from  fixed 
terminals  on  the  frame  of  the  machine,  to  which  the  ends  of  the 
armature  wires  are  brought.  In  this  form  of  machine  the  main 
lines  of  construction  are  very  similar  to  that  of  the  multipolar 
continuous  current  machine,  in  many  respects.  There  is  the  same 
enclosing  cylinder,  or  ring  of  iron  or  steel,  fixed  on  its  bedplate,  the 
bedplate  carrying  the  pedestals  for  the  bearings  of  the  axle  of  the 
machine,  as  before.  On  the  inner  side  of  the  enclosing  ring,  and 
cast  into  the  ring  in  some  forms  of  machine,  securely  held  to  it  in 
all  forms,  are  thin  discs  of  iron,  having  slots  cut  on  their  inner  edges, 
the  slots  forming,  when  the  discs  are  built  into  the  ring,  longitudinal 
channels  in  which  the  wires  are  placed.  The  armature  coils  are  made 
on  formers,  very  much  in  the  same  manner  as  described  for  the 
armature  coils  of  continuous  current  machines,  their  cotton  coverings 
are  dried  in  a  vacuum  oven,  are  then  steeped  in  insulating  varnish, 
dried,  wrapped  with  tapes  dried,  and  fixed  in  their  positions  in  the 
slots  in  the  armature  cores,  these  having  been  previously  carefully 
insulated  by  troughs  of  mica,  micanite,  presspahn,  or  other  suitable 
material.  The  field  magnet  cores  are  secured  to  an  iron  ring  or  a 
flywheel  held  on  the  driving  axle,  the  exciting  coils  being  slipped 
over  the  cores,  the  coils  having  been  prepared  and  insulated  in  a 
similar  manner  to  that  described  with  continuous  current  machines, 


THE   GENERATION   OF   ELECTRICITY  189 

and  being  held  in  place  by  the  pole  pieces,  which  are  bolted  to  the 
field  magnet  cores,  or  the  whole  thing,  or  its  pole  piece  with  the  field 
coil  slipped  over  the  core,  may  be  bolted  to  a  projection  on  the 
revolving  disc.  The  poles  of  the  field  magnet,  as  before,  are  arranged 
north,  south,  north,  south,  all  the  way  round,  and  the  operation  of 
the  machine  is  identically  the  same  as  those  previously  described, 
the  field  magnet  cores  bringing  the  lines  of  force  to  the  armature 
coils,  and  sweeping  them  across  the  coils  in  place  of  the  coils  being 
swept  across  the  lines  of  force.  The  excitor  dynamo  in  this  form 
of  machine  is  usually  carried  on  an  extension  of  the  frame  of  the 
machine,  either  of  the  bedplate  or  the  pedestal  carrying  one  of 
the  bearings  for  the  axle,  the  axle  of  the  excitor  being  mechanically 
connected  to  the  axle  of  the  revolving  field  magnets  of  the  alternator, 
as  described  in  connection  with  other  machines.  Plates  SA,  B,  and  c 
show  a  revolving  field  alternator  for  connecting  to  a  steam  turbine. 


Single,  Two,  and  Three  Phase  Alternators 

The  alternators  that  have  been  described  all  generate  one  current, 
rising,  falling,  and  reversing,  as  explained,  and  are  known  as  single- 
phase  alternators.  But  single-phase  alternating  motors,  not  yet 
having  reached  the  state  required  for  mining  work,  single-phase 
alternating  currents  have  not  been  used,  and  are  not  yet  suitable 
for  mining  work.  Two-phase  and  three-phase,  principally  the  latter, 
forms  are  employed.  In  the  two-phase  alternating  current  generator, 
as  explained  in  Chapter  I.,  there  are  two  distinct  alternating  currents 
generated  in  the  same  machine,  at  one  operation,  by  the  revolution 
of  one  armature,  or  set  of  field  magnets,  the  currents  succeeding  each 
other  by  a  quarter  of  an  alternating  current  period.  In  the  disc 
machine  the  two  sets  of  currents  require  two  sets  of  coils,  which 
must  be  spaced  on  the  disc  so  that  they  succeed  each  other  by  the 
interval  named,  the  second  set  of  coils  just  entering  the  fields  when 
the  first  set  are  at  the  point  of  maximum  strength.  With  three- 
phase  currents  in  the  disc  machine  there  must  be  three  sets  of  coils 
spaced  so  that  their  pressures  and  currents  follow  each  other  by 
one-third  of  a  period,  the  second  set  just  entering  the  fields  when 
the  first  set  has  passed  through  the  point  of  maximum  strength  of 
field,  and  has  reached  one-third  of  the  distance  towards  the  point 
where  the  field  is  nil,  the  third  set  following  the  second  set  at  the 
same  distance  as  the  second  set  follows  the  first,  and  so  on. 

In  the  alternator  with  a  drum  armature  the  only  addition  that 
is  necessary  for  generating  two-phase  currents  is,  a  second  pair  of 
collecting  rings  are  fixed  on  the  driving  axle,  insulated  from  the 
first  pair,  and  from  each  other,  with  a  second  pair  of  collecting 


190 


ELECTRICITY  IN   MINING 


brushes  bearing  upon  them,  the  second  pair  of  rings  being  connected 
to  points  in  the  winding  also  at  opposite  ends  of  a  diameter,  this 
diameter  being  at  right  angles  to  the  diameter  at  which  connection 
is  made  for  the  first  phase.  By  this  arrangement  two  currents  are 

delivered,  exactly  similar  in 
character,  but  following  each 
other  at  90°  interval,  or  a  quarter 
of  a  period.  This  is  shown 
diagrammatically  in  Fig.  88. 
For  generating  three  -  phase 
currents,  three  collecting  rings 
are  fixed  on  the  driving  axle, 
with  brushes  bearing  on  them, 
the  rings  being  connected  to 
three  points  on  the  armature 
120°  apart.  In  the  revolving 
field  alternator  for  two-phase 
currents  there  are  two  sets  of 
coils  fixed  in  the  slots  in  the 
armature  disc,  on  the  inner  side 
of  the  containing  cylinder,  and 
for  three-phase  currents  there  are 
three  sets  of  coils  held  in  the  slots. 

The  four  ends  of  the  two-phase  coils  are  brought  out  to  four  terminals 
fixed  on  any  convenient  part  of  the  machine,  insulated  from  each 
other  and  from  the  machine,  and  to  these  terminals  the  conductors 
leading  to  the  outer  circuit  are  connected.  For  three-phase  currents 


FIG.  88. — Diagram  of  Connections  of 
a  Two-phase  Drum  Armature.  1  and 
3  are  the  Connections  for  one  Phase, 
2  and  4  for  the  other. 


FIG.  89. — Diagram  of  Connections  of  Three-phase  Armatures.    The  A  is  the 
Closed  Coil  or  Drum  Armature. 

there  are  two  methods  of  connecting,  known  respectively  as  "  star  " 
and  "delta"  windings,  the  latter  being  sometimes  called  "mesh" 
winding.  With  "  star  "  winding  one  end  of  each  of  the  three  coils 
are  connected  together,  the  junction  of  the  three  forming  what  is 


THE  GENERATION   OF  ELECTRICITY  191 

termed  the  neutral  point,  the  other  end  of  each  of  the  coils  being 
brought  out  to  three  terminals,  insulated  from  each  other  and  from 
the  machine,  fixed  at  any  convenient  point,  as  before,  and  to  these 
terminals  the  cables  for  the  service  are  connected.  The  connections 
of  the  three-phase  system  in  the  drum  armature  alternator  is  the 
best  example  of  "  delta "  or  "  mesh "  connection.  The  armature 
coils  of  a  three-phase  revolving  field  alternator  may,  however,  be 
connected  in  this  way.  Diagrams  of  the  connections,  with  "  Delta  " 
and  "  Star  "  arrangement,  are  shown  in  Fig.  89. 


The  Output   and    Number    of    Poles  of  Single, 
Two,   and  Three   Phase  Alternators 

The  number  of  poles  required  by  any  alternator  is  the  same, 
whether  it  be  for  single,  two,  three,  or  more  phases,  and  it  depends 
simply  upon  the  periodicity,  that  is  to  say,  the  number  of  cycles,  and 
the  speed  at  which  the  machine  is  to  run.  The  number  of  cycles  means 
the  number  of  reversals  the  machine  is  required  to  produce,  and  is 
obtained  for  any  given  machine  by  taking  the  number  of  magnetic 
fields,  that  is  to  say,  half  the  number  of  single  poles,  and  multiplying 
by  the  number  of  revolutions  per  minute.  With  a  periodicity  of  50 
per  second,  which  equals  3000  per  minute,  if  a  machine  is  to  run  at, 
say,  500  revolutions  per  minute,  it  must  have  6  magnetic  fields,  or  12 
poles.  With  a  periodicity  of  only  25  and  the  same  speed,  half  the 
number  of  poles  would  be  sufficient.  On  the  other  hand,  with  lower 
speed,  say  in  the  case  of  a  large  flywheel  alternator  running  at  100 
revolutions  per  minute  with  a  periodicity  of  50  cycles  per  second, 
there  must  be  30  magnetic  fields,  or  60  magnetic  poles.  And  this 
number  is  required  whether  the  machine  is  furnishing  one,  two, 
or  three  currents. 

The  output  of  a  two-phase  machine  is  larger  than  that  of  a  single- 
phase  machine,  while  the  output  of  a  three-phase  machine  is  the 
same  as  that  of  a  two-phase.  The  proportion  between  the  output  of 
any  given  machine,  as  single-phase  and  as  two  or  three  phase,  is  as 
approximately  65  to  100,  the  single-phase  machine  only  generating 
65  per  cent,  of  the  energy  generated  by  the  two  or  three  phase 
machine.  With  the  two-phase  machine  the  output  is  measured  by 
the  product  of  the  highest  virtual  or  effective  pressure,  multiplied  by 
the  highest  virtual  or  effective  current  obtainable  in  one  phase,  from 
the  machine  at  the  same  moment,  the  product  being  multiplied  by  2. 
With  three-phase  machines  the  output  is  measured  by  the  product 
of  the  highest  effective  current  in  any  phase,  multiplied  by  the 
highest  effective  pressure  when  the  highest  virtual  current  is  passing, 
the  product  being  multiplied  by  \/3  =  1*71. '  There  is  a  difference 


192  ELECTRICITY  IN   MINING 

in  the  arrangements  of  the  pressures  with  the  star  connection  and 
mesh  connection,  though  it  does  not  affect  the  total  output  of  the 
machine.  With  star  connection  the  pressure  between  any  two 
terminals  of  the  machine  is  1*71  times  the  pressure  between  any 
terminal  and  the  neutral  point,  the  junction  of  the  inner  ends  of  the 
three  sets  of  coils.  In  mesh  connection  the  current  passing  out 
to  the  outer  circuit  through  either  of  the  cables  is  1/71  times  the 
current  passing  in  the  coils  connected  between  any  two  terminals. 


The  Inductor  Alternator 

In  this  form  of  machine,  which  has,  so  far  as  the  author  is  aware, 
not  been  much  employed,  the  whole  of  the  conductors  are  stationary, 
and  the  changes  in  the  number  of  lines  of  force  passing  through  the 
armature  coils  is  brought  about  by  changes  in  the  magnetic  circuit  of 
the  machine.  It  was  explained  in  Chapter  I.  that  the  number  of 
lines  of  force  passing  through  any  magnetic  field,  and,  in  particular, 
passing  across  the  small  gap  allowed  for  the  moving  coils  of  a 
generator,  depends  directly  upon  the  exciting  force  measured  by  the 
product  of  the  current  in  amperes,  multiplied  by  the  number  of  turns 
the  current  made  round  the  field  magnet  cores,  and  inversely  on  the 
resistance  or  reluctance  of  the  magnetic  circuit  in  which  the  air 
space,  or  the  space  left  for  the  armature  coils,  forms  a  part.  In  the 
disc,  the  drum,  and  the  revolving  field  type  of  alternators,  the  reluc- 
tance of  the  magnetic  field  is  changed  by  varying  the  position  of  the 
armature  core  and  the  field  magnet  cores  with  reference  to  each 
individual  coil,  this  leading  to  a  change  in  the  number  of  lines  of 
force  passing  through  each  coil,  and  to  the  creation  of  an  electrical 
pressure  in  consequence.  In  the  inductor  alternator  the  change  in 
the  magnetic  reluctance  is  produced  by  motion  of  a  portion  of  the 
iron  forming  the  magnetic  circuit  itself.  In  the  inductor  alternator 
the  armature  coils  are  fixed,  and  the  exciting  coils  are  also  fixed, 
both  being  held  usually  in  positions  on  the  inner  side  of  an  enclosing 
iron  or  steel  ring,  similar  to  that  employed  in  the  multipolar  con- 
tinuous current  generator,  and  the  other  alternators  that  have  been 
described.  The  position  of  the  exciting  coils  of  any  magnetic  circuit 
is  immaterial  within  certain  limits.  They  may  be  placed  in  any 
convenient  position,  providing  that  they  create  the  necessary  lines  of 
force  in  the  direction  required  for  the  operation  of  the  machine. 
The  armature  coils  must  be  in  such  a  position  that  any  changes 
created  in  the  number  of  lines  of  force  passing  through  the  field  must 
take  place  in  them ;  that  is  to  say,  the  full  changes  in  the  number 
of  lines  of  force  must  take  place  in  the  portion  of  the  magnetic 
circuit  occupied  by  each  individual  armature  coil.  In  the  inductor 


o>  a, 

£  •  I 

o  §  ^ 

il  s 


' 


THE   GENERATION    OF   ELECTRICITY  193 

machine  there  are  a  number  of  magnetic  poles,  north,  south,  north, 
south,  arranged  around  the  enclosing  ring,  just  as  in  the  other 
machines ;  and  it  may  also  be  split  up  into  a  number  of  magnetic 
circuits,  each  consisting  of  the  portion  of  the  outer  containing  ring 
carrying  a  north  and  a  south  pole,  with  the  connecting  yoke,  and  a 
portion  of  the  iron  armature,  as  it  is  practically,  carried  by  the 
revolving  member.  The  revolving  portion  of  the  apparatus  consists 
of  the  axle  and  an  iron  wheel  with  iron  projections,  and  as  the  axle 
revolves,  the  iron  projections  approach  the  poles  formed  on  the  inner 
side  of  the  enclosing  ring,  very  much  as  the  field  magnet  cores  of  the 
revolving  type  do,  the  approach  of  the  cores  to  the  poles  reducing  the 
magnetic  resistance,  and  their  recession  increasing  its  resistance, 
the  result  being  an  increase  of  lines  of  force  and  an  increase  of 
pressure  as  the  projecting  iron  comes  up  to  the  field  magnet  pole,  and 
a  decrease  of  lines  of  force  and  a  decrease  of  pressure  as  it  recedes. 
The  necessary  reversal  of  connections  required  to  make  the  pressure, 
delivered  to  the  outer  circuit  all  in  the  same  direction,  is  arranged  by 
having  two  sets  of  armature  coils  and  two  sets  of  pole  pieces,  but 
with  only  one  set  of  revolving  projections. 


Secondary  Batteries  or  Accumulators 

Secondary  batteries  have  not  yet  been  much  employed  in  mines, 
but  where  they  can  be,  they  form  another  source  of  economy  under 
certain  conditions.  The  secondary  battery  or  accumulator  is  also 
known  as  a  storage  battery,  from  the  fact  that  electricity  is  poured 
into  it  from  a  dynamo,  and  that  afterwards  electricity  can  be  taken 
from  it  for  a  certain  period.  There  are  two  forms  of  secondary 
battery  on  the  market,  though  one  of  them  has  hardly  attained 
commercial  success  as  yet,  viz.  the  lead  lead-oxide  battery,  and  the 
iron  nickel  battery  that  has  been  worked  out  by  Mr.  Edison.  In 
every  secondary  battery  there  are  practically  three  distinct  portions, 
the  electrodes  or  carriers,  the  active  material,  and  the  liquid  or 
electrolyte.  In  the  lead  lead-oxide  battery  the  electrodes  are  grids 
of  lead.  In  some  of  the  later  forms  a  lead  antimony  alloy  is 
employed  to  give  greater  stiffness  to  the  grid,  and  therefore  to  allow 
of  its  being  made  lighter.  The  grids  are  arranged  in  various  forms, 
but  all  designed  to  have  cavities  formed  in  them  for  the  active 
material,  of  such  a  nature  that  in  the  course  of  charge  and  discharge, 
during  which  considerable  expansion  and  contraction  of  the  active 
material  takes  place,  the  tendency,  which  is  only  too  common,  of  the 
active  material  to  chip  off,  shall  be  reduced  to  the  lowest  possible 
limits.  In  the  lead  lead-oxide  battery  the  active  material  consists  of 
two  oxides  of  lead ;  a  low  oxide  is  placed  on  the  negative  plate,  and  a 

o 


i94  ELECTRICITY    IN   MINING 

higher   oxide   on   the   positive  plate.      In   some   forms   the   active 
material  of  the  positive  plate  is  formed  by  electro-chemical  action, 
out  of  the  mass  of  the  plate  itself,  by  what  is  known  as  the  Plante 
method.     The  grids   with  their  active  material  are  suspended  ver- 
tically in  glass  or  lead-lined  wooden  vessels  in  dilute  sulphuric  acid. 
The  current  from  a  continuous-current  dynamo  is   brought  to  the 
positive  plate,  it  passes  through  the  liquid  to  the  negative  plate,  and 
thence  back  to  the  dynamo.     In  its  passage  it  oxidizes  the  higher 
oxide  upon  the  positive  plate,   raising  it  to  a  yet  higher  oxide ;  it 
decomposes  a  portion  of  the  dilute  sulphuric  acid,  the  oxygen  passing 
to  the  positive  plate,  and  forming  the  source  from  which  the  positive 
active  material  is  oxidized ;  and  it  reduces  the  lower  oxide  upon  the 
negative  plate  to  the  form  of  lead,  in  a  spongy  condition,  the  hydrogen 
from  the  decomposition  of  the  dilute  sulphuric  acid  combining  with 
the  oxygen  liberated  from  the  negative  active  material  to  form  water. 
It  may  be  said,  in  fact,  that  the  operation  of  charging  a  secondary 
battery  consists  in  the  transference  of  a  certain  quantity  of  oxygen 
from  the  negative  active  material  to  the  positive  active  material. 
The  higher  oxide  of  lead  formed  on  the  positive,  and  the  spongy  lead 
formed   on   the  negative  plate,  in  the  presence  of  the  solution  of 
sulphuric  acid,  which  has  become  considerably  stronger  during  the 
process  of  charging,  forms  the  most  powerful  galvanic  couple  that  is 
known.     The  electrical  pressure  between  its  two  plates  when  first 
charged,  and  when  the  gas  which  is  present  and  which  has  not  been 
absorbed  by  the  active  materials  has  passed  off,  is  two  volts.     When 
the  secondary  battery  discharges,  the  reverse  operations  take  place ; 
oxygen  is  transferred  from  the  active  material  of  the  positive  plate, 
via  the  solution   of  sulphuric   acid,  to   the  active   material  of  the 
negative  plate.     The  active  material  on  the  positive  plate  is  reduced 
to  the  oxide  from  which  it  was  formed  by  the  charging  current,  the 
active  material  on  the  negative  plate  is  re-oxidized  to  the  form  from 
which  it  was  reduced  by  the  charging  current,  and  the  liquid  electro- 
lyte recovers   the  water  that  was  taken   from  it  by  the  charging 
current,  the  strength  of  the  solution  again  decreasing. 

In  the  Edison  secondary  battery  the  electrodes  or  carrier  plates 
are  thin  plates  of  nickel  steel,  out  of  which  rectangular  spaces  have 
been  punched,  and  into  these  spaces  perforated  boxes,  also  of  nickel 
steel,  are  forced  under  hydraulic  pressure,  so  that  the  boxes  and  the 
carrier  plate  form  one  homogeneous  mass.  The  perforated  steel  boxes 
contain  a  salt  of  iron  in  a  finely  divided  state  on  one  plate,  and  a 
mixture  of  a  salt  of  nickel  and  carbon,  both  in  a  finely  divided  state, 
on  the  other  plate.  The  electrolyte  is  caustic  potash.  The  cell, 
when  fully  charged,  has  a  pressure  between  its  terminals  of  only 
1-6  volts. 

Secondary  cells  are  usually  made  up  with  a  certain  number  of 


THE  GENERATION   OF  ELECTRICITY  195 

plates  in  each  cell  of  a  certain  size,  the  sizes  of  the  cells  being 
arranged  according  to  the  work  they  are  intended  to  perform,  and 
their  capacity  is  given  as  so  many  ampere  hours ;  that  is  to  say,  a 
cell  with  a  certain  number  of  plates,  each  of  a  certain  size,  will  furnish 
a  current  of  a  certain  strength  for  a  certain  number  of  hours.  The 
capacity  of  secondary  cells  varies  with  the  period  of  discharge.  It  is 
greatest  for  long  periods,  such  as  ten  hours,  and  it  becomes  less  as 
the  strength  of  the  current  taken  from  the  cell  increases,  and  the 
period  of  discharge  decreases.  Thus,  with  a  cell  listed  as  having 
a  capacity  of  150  ampere  hours,  this  will  mean  usually  that  it  will 
give  fifteen  amperes  for  ten  hours. 

If  the  rate  of  discharge  is  raised,  say,  to  twenty  amperes,  the 
total  capacity  will  be  reduced  to  120  ampere  hours,  or  thereabouts, 
and  it  will  be  reduced  still  further  if  the  rate  of  discharge  is  further 
increased.  When  a  secondary  battery  is  being  charged,  the  gases 
which  are  formed  by  the  decomposition  of  the  liquid  electrolyte,  add 
to  the  back  pressure  created  within  the  cell,  which  has  to  be  over- 
come by  the  charging  current.  With  a  lead  lead- oxide  battery,  each 
cell  when  charging  has  a  back  pressure  of  2  J  volts,  and  the  charging 
current  must  have  that  pressure,  and,  in  addition,  a  pressure  sufficient 
to  overcome  the  resistance  of  the  cell  itself,  that  offered  by  the 
electrolyte,  the  plates,  and  what  is  known  as  the  contact  resistance 
between  plates  and  the  electrolyte.  The  resistance  of  accumulators 
is  usually  very  small  compared  with  that  of  primary  batteries,  and 
the  additional  pressure  required  to  overcome  the  resistance  of  the 
cell  is  not  great,  but  it  must  be  provided  for. 

From  the  moment  that  the  charging  current  ceases  to  pass  through 
the  accumulator,  or  if  the  pressure  of  the  charging  current  falls  below 
that  necessary,  as  described  above,  the  pressure  of  the  cells  commences 
to  fall.  The  gases  which  give  the  additional  half- volt  pressure  usually 
quickly  disperse,  or  recombine,  and  the  pressure  of  the  cell  falls  to  2 
volts,  or  in  some  cases  a  little  over,  2*1  say,  very  quickly.  The  pressure 
of  the  secondary  cell  also  commences  to  fall,  apart  from  the  escape  of 
the  gases,  after  the  charging  current  has  ceased  to  pass  through  it. 
When  current  is  being  taken  from  it,  the  pressure  falls  approximately 
in  proportion  to  the  strength  of  the  current,  and  in  practical  work 
no  cell  should  ever  be  allowed  to  fall  below  a  pressure  of  1*8  volts, 
before  it  is  recharged.  Where  cells  are  not  used  they  should  be 
carefully  tested  and  watched,  and  given  an  occasional  charge  to  keep 
them  up  to  their  full  pressure.  Leakage  takes  place  with  all  bat- 
teries, both  primary  and  secondary.  A  connection  exists  between  the 
terminals  of  adjacent  cells,  both  primary  and  secondary,  by  way  of 
the  moisture  which  is  nearly  always  present  on  the  outside  of  the 
cell,  and  upon  the  box  or  shelf  on  which  they  rest,  and  this  connec- 
tion, though  it  has  a  path  of  high  resistance,  allows  a  leakage  current 


196  ELECTRICITY   IN    MINING 

to  pass,  which  is  always  going,  and  which  gradually  works  the  cells 
down.  With  secondary  batteries  it  is  usual  to  support  the  cells  upon 
insulators  of  porcelain,  arranged  so  that  the  leakage  current  has  to 
pass  over  the  surface  of  a  film  of  oil,  this  introducing  a  very  high 
resistance  into  its  path.  Secondary  batteries  also  should  never  be 
allowed  under  any  circumstances  to  be  fully  discharged.  The  rule 
given  above  as  to  not  allowing  them  to  go  below  a  pressure  of  1*8 
provides  for  this.  If  they  are  allowed  to  go  below  that,  action  takes 
place  between  the  lead  grid  and  the  active  material,  resulting  in  the 
formation  of  lead  sulphate  upon  the  surface  of  the  lead  grid,  the 
sulphate  having  a  very  high  resistance,  and  practically  in  course  of 
time  stopping  the  passage  of  both  charging  and  discharging  current, 
and  rendering  the  cell  useless.  The  sulphate  is  afterwards  formed 
into  active  material  by  the  charging  current,  when  the  cell  is  put 
right,  but  it  is  at  the  expense  of  the  substance  of  the  plate,  the  latter 
gradually  becoming  brittle  and  breaking  up,  this  being  one  of  the 
great  troubles  in  connection  with  secondary  batteries,  where  they  are 
not  properly  looked  after. 

Accumulators  are  employed,  in  generating  stations,  to  assist  the 
generators  at  times  of  heavy  load,  the  accumulators  taking  up  the 
surplus  power  during  times  of  light  load,  just  as  the  thermal  store 
does  with  the  Eateau  turbine.  In  order  to  charge  them,  however, 
from  the  regular  supply  service,  the  pressure  has  to  be  raised  for  the 
purpose  by  a  "  Booster,"  as  described  below.  They  can  only  be  used, 
at  present,  with  continuous  currents. 


Boosters 

The  booster  is  an  apparatus,  as  its  name  will  probably  make 
known,  that  has  been  introduced  from  America.  It  is  a  motor 
generator,  and  its  office  is  to  "  boost,"  or  to  increase,  and  in  certain 
cases  to  decrease,  the  pressure  delivered  by  the  generators  at  any  part 
of  the  distribution  cables.  It  is  also  employed  for  providing  the 
necessary  increase  of  pressure  for  charging  accumulators  from  the 
ordinary  generators  employed  in  the  generating  station.  Where 
accumulators  are  employed  to  assist  the  ordinary  supply  service, 
they  must  furnish  current  at  the  same  pressure  as  that  furnished  by 
the  generators,  and  it  is  evident  that  they  cannot  be  charged  with  a 
pressure  of  something  above  2J  volts  per  cell  from  the  generators 
direct,  and  give  the  generator  pressure  when  called  upon.  The 
booster  overcomes  the  difficulty.  In  this  case  it  consists  of  two 
machines,  or  it  may  consist  of  one  machine  with  two  windings  on  its 
armature,  and  two  commutators.  Where  it  consists  of  two  machines, 
one  machine  receives  current  as  a  motor  from  the  supply  service, 


THE   GENERATION    OF   ELECTRICITY  197 

driving  the  other  machine  as  a  generator,  and  the  second  machine 
generates  current  at  sufficient  pressure  to  charge  the  accumulators, 
the  pressure  of  the  current  generated  being  regulated  by  the  current 
in  the  field  magnet  coils.  Where  a  single  machine  with  two  wires 
on  its  armature  is  employed,  the  field  magnets  and  one  winding  of 
the  armature  receive  current  from  the  generators,  the  other  winding 
furnishing  current  at  the  pressure  required  for  charging  the  accumu- 
lators. Where  the  accumulator  is  employed  to  assist  the  generators 
in  taking  what  is  called  the  "  peak  of  the  load,"  the  portion  of  the 
load,  where  it  exists,  which  only  comes  on  at  a  certain  portion  of  the 
day,  and  only  for  a  short  time,  and  is  over  and  above  the  ordinary 
load  of  the  station,  the  accumulators  are  sometimes  allowed  to 
discharge  through  the  booster  into  the  supply  service,  the  booster  in 
this  case  reducing  the  accumulator  pressure  to  that  of  the  supply 
service,  the  accumulator  current  only  being  employed  for  a  short 
period.  During  recent  years  there  has  been  a  development  of  this, 
known  as  the  "  reversible "  booster,  which  is  always  connected  to 
the  accumulators,  and  to  the  generator  service.  With  this  arrange- 
ment the  generators  and  the  engines  driving  them  are  made  of  less 
power  than  would  otherwise  be  necessary,  and  are  always  furnishing 
current  right  up  to  their  full  output.  Whenever  there  is  a  margin 
between  the  current  taken  by  the  supply  service  and  that  furnished 
by  the  generators,  it  is  taken  up  by  the  accumulator  through  the 
booster  ;  and  whenever  the  demand  of  the  supply  service  is  more  than 
that  able  to  be  furnished  by  the  generators,  the  difference  is  made  up 
by  the  accumulators  again  through  the  booster.  In  practice  the 
reversible  booster  is  constantly  either  transmitting  current  to  the 
accumulator,  or  from  the  accumulator  to  the  supply  service,  and 
the  arrangement  has  resulted  in  a  large  increase  in  the  efficiency  of 
the  accumulator.  In  ordinary  work  the  efficiency  of  the  accumulator 
cannot  be  taken  at  more  than  70  per  cent.,  more  frequently  it  is  less, 
as  it  must  not  be  discharged  below  a  pressure  of  1'8  per  cell.  The 
efficiency  of  the  booster  cannot  be  taken  at  more  than  80  per  cent., 
so  that  the  combined  efficiency  will  not  exceed  55  to  60  per 
cent.  It  is  claimed  that  with  some  forms  of  reversible  booster, 
efficiencies  as  high  as  80  per  cent.,  inclusive  of  accumulator  and 
booster,  have  been  obtained,  this  being  due,  in  the  author's  opinion, 
to  the  fact  that  the  pressure  of  the  gases  generated  in  the  course  of 
charging  the  accumulator  is  made  use  of  to  a  certain  extent,  instead 
of  being  lost  in  the  ordinary  arrangement  for  charging  accumulators. 


ELECTRICITY   IN   MINING 


Motor  Generators 

The  motor  generator  has  been  referred  to  in  Chapter  III.,  and  in 
connection  with  boosters.  As  explained,  it  consists  of  two  machines, 
one  of  which  receives  current  as  a  motor,  and  the  other  generates 
current.  The  two  machines  may  be  continuous-current  machines,  or 
one  may  be  arranged  for  alternating  current,  while  the  other  is 
arranged  for  continuous  current.  The  two  machines  are  always 
arranged  to  deal  with  the  same  quantity  of  energy,  and  the  office  of 
the  combined  machine  is  to  convert  the  electrical  energy,  say,  of  a 
power  service  from  one  form  to  another.  In  the  simplest  form  there 
are  two  identically  similar  continuous-current  generators,  usually  of 
the  two-pole  type,  though  with  the  development  of  dynamo  con- 
struction four-pole  machines  are  becoming  common,  even  for  small 
sizes.  They  are  mounted  on  one  bedplate,  and  may  be  complete  in 
themselves,  each  being  capable  of  removal,  or  the  bedplate  may  form 
part  of  the  combined  machine,  the  two  sets  of  field  magnets  being 
secured  to  it,  or  even  forming  part  of  the  same  casting.  Each  machine 
has  its  own  field  magnets,  its  own  armature,  its  own  commutator,  and 
its  own  brushes  and  terminals.  There  will  be  two  bearings  carried 
on  pedestals  fixed  to  the  bedplate  at  the  ends  of  the  combined 
machines,  but  there  is  usually  no  third  bearing  between  them.  The 
axles  of  the  two  armatures  are  connected  together  in  the  space 
between  the  two  machines,  and  they  revolve  together.  The  two 
machines  are  wound  for  different  pressures,  one  being  wound  to 
receive  current  at  the  pressure  of  the  supply  service,  from  which  it 
is  to  convert,  and  the  other  being  wound  to  generate  current  at  the 
pressure  desired.  The  motor  may  also  be  series,  shunt,  or  compound 
wound,  but  is  more  usually  shunt  wound,  though  it  may  have  a  series 
coil  to  assist  it  in  starting.  The  generator  may  be  also  series,  shunt, 
or  compound  wound,  but  it  will  be  more  usually  shunt  or  compound. 
A  modification  of  this  arrangement  that  is  made  by  some  firms  is 
also  a  modification  of  the  machine  described  on  p.  200  for  use  with 
the  three-wire  distribution  system.  It  is  known  as  the  Dynamotor. 
It  has  one  pair  of  field  magnets  only,  one  armature  only,  but  the 
armature  has  two  windings,  two  commutators,  and  two  sets  of 
brushes.  The  field  magnets  are  excited  by  the  supply  current,  and 
one  of  the  windings  on  the  armature  receives  current  and  turns  the 
armature  in  the  field  created  by  the  field  magnets,  as  a  motor,  the 
other  winding  on  the  armature  generating  a  current  at  any  pressure 
that  may  be  desired.  The  arrangement,  as  before,  converts  a  con- 
tinuous current  of  any  pressure  to  a  continuous  current  of  any  other 
pressure.  It  should  not  be  employed  for  pressures  which  differ  to 
any  great  extent,  and  it  has  the  disadvantage,  as  against  the  motor 


THE   GENERATION   OF  ELECTRICITY  199 

generator  with  two  distinct  machines,  that  there  is  an  electrical 
connection  between  the  wires,  cables,  etc.,  at  the  two  pressures 
through  the  insulation  between  the  coils  of  the  armature,  whereas  there 
is  no  electrical  connection  between  the  two  in  the  motor  generator 
proper,  and  there  is  not  much  space  with  a  double  winding  to  provide 
insulation,  where  one  of  the  pressures  is  comparatively  high.  On 
the  other  hand,  the  arrangement  is  cheaper  in  first  cost  than  two 
machines. 

Another  arrangement  is,  the  motor  may  be  arranged  to  work  with 
two  or  three  phase  currents,  and  the  generator  to  create  continuous 
current  pressures.  As  before,  the  machines  are  of  the  same  output, 
and  this  enables  continuous  currents  to  be  supplied  at  any  convenient 
pressure,  say,  for  signals  for  incandescent  lamps  or  for  arcs,  from  a 
two  or  three  phase  power  service,  no  matter  what  the  pressure  of  the 
power  service  may  be.  If  the  pressure  of  the  power  service  is  high, 
it  is  a  simple  matter  to  transform  it  down  by  means  of  the  stationary 
transformers  to  be  described,  to  a  conveniently  low  pressure,  at  which 
it  can  be  delivered  to  the  motor.  The  reverse  arrangement  may  also 
be  made,  the  motor  machine  may  be  for  continuous  current,  and  the 
generator  for  alternating,  which,  again,  may  be  for  single,  two,  or  three 
phases,  as  convenient.  A  motor  generator  for  transferring  three  phase 
to  continuous  currents  is  shown  in  Plate  TB. 

There  is  yet  another  arrangement,  designed  particularly  for  con- 
verting alternating  to  continuous  currents,  or  continuous  currents 
to  alternating,  known  as  the  "  Eotary  Converter."  The  rotary 
converter,  as  usually  constructed,  is  a  drum-wound  alternator.  It 
may  also  be  looked  upon  as  a  multipolar  continuous-current 
dynamo  with  a  drum-wound  armature.  The  armature  has  the  usual 
commutator  for  continuous  currents  at  one  end,  and  at  the  other  end 
it  carries  two,  three,  or  four  collector  rings,  with  brushes  bearing  upon 
them,  as  in  the  alternating-current  machines,  the  collector  rings  being 
connected  to  points  180,  120,  or  90  degrees  apart  on  the  armature,  as 
in  the  drum-wound  alternator,  according  as  single-phase,  three-phase, 
or  two-phase  currents  are  required.  There  is  one  important  feature 
in  connection  with  the  rotary  converter:  the  pressures  bear  a 
certain  definite  relation  to  each  other,  the  alternating  current  pressure 
being  always  less  than  the  continuous  current  pressure,  the  ratios 
being,  with  single  phase,  707  per  cent.,  with  two  phase,  70'7  per  cent., 
and  with  three  phase,  61*2  per  cent,  of  the  continuous  current  pressure. 
If  a  continuous  current  is  delivered  to  the  motor  side  at  100  volts, 
the  single  or  two  phase  current  generated  is  70*7  volts,  and  the  three 
phase  61*2  volts,  while  a  motor  pressure  of  70*7  two  phase,  or  61 '2 
volts  three  phase  produce  100  volts  continuous  on  the  generator  side. 
The  current  is  increased  or  decreased  in  proportion.  Thus  with  100 
volts  continuous  motor  pressure,  70*7  volts  two-phase  currents  are 


200  ELECTRICITY  IN   MINING 

generated,  of  70'7  amperes  in  each  phase,  and  three  currents  of  94'4 
amperes  with  three  phase. 

Eotary  convertors  may  be  employed  in  the  same  manner  as 
either  the  continuous  current  machine  or  the  alternating  machine 
would  be  if  steam  driven,  but  certain  care  has  to  be  taken  in  con- 
nection with  them.  A  sub-station,  consisting  of  rotary  convertors, 
is  shown  in  Plate  9  A. 

Balancers 

The  balancer  is  an  apparatus,  as  will  be  explained  in  Chapter  V., 
that  is  employed  in  the  distribution  of  current  on  what  is  called  the 
three-wire  system.  It  is  really  a  continuous-current  motor  generator, 
in  which  the  two  machines  are  identically  alike,  and  it  is  sometimes 
arranged,  as  will  be  explained,  for  one  machine,  receiving  current  from 
one  portion  of  the  service,  to  drive  the  other  machine,  which  generates 
current  for  delivery  to  the  other  portion  of  the  service.  Either 
machine  may  act  as  the  motor  at  times,  the  other  machine  acting 
then  as  the  generator,  and  either  machine  may  act  as  generator  when 
the  other  one  acts  as  motor.  The  two  machines  may  also  be,  and  are 
frequently  in  the  latest  practice,  driven  by  a  steam  engine.  In  this 
case  each  machine  acts  as  a  generator,  only  delivering  current,  the 
pressure  of  which  is  controlled  by  apparatus  that  will  be  described, 
to  different  parts  of  the  service. 

Continuous-current  Machines  with  Two 
Armature  Windings 

Continuous  current  machines  are  made  by  a  few  firms,  though, 
the  author  understands,  not  in  large  sizes,  with  two  identically 
similar  windings  on  the  armature,  and  two  commutators,  one  at  each 
end.  The  machines  are  separately  excited,  this  being  the  more  con- 
venient arrangement,  though  it  is  not  absolutely  necessary.  The 
object  of  this  form  of  machine,  as  will  be  explained  in  Chapter  V.,  is 
for  use  with  the  three- wire  system  of  distribution.  The  two  windings 
of  the  armature  are  connected  in  series,  leaving  the  positive  end  of 
one  and  the  negative  end  of  the  other  as  the  terminals  of  the  machine, 
the  middle  or  joint  terminal  forming  the  middle  or  neutral  terminal 
of  the  three- wire  system. 

Stationary  Transformers 

The  stationary  transformer  is  a  device  that  has  been  very  much 
employed  in  the  distribution  of  electric  currents  for  both  power  and 
lighting  by  alternating  currents.  It  is  a  development  of  the 


THE   GENERATION   OF  ELECTRICITY  201 

induction  coil,  the  coil  that  is  employed  for  medical  use,  and  for 
X-ray  apparatus,  and  similar  arrangements.  Its  useful  property  is 
the  ability  to  convert  a  current  of  low  pressure  to  one  of  high  pressure, 
or  a  current  of  high  pressure  to  one  of  low  pressure. 

In  the  ordinary  induction  coil  there  is  a  straight  iron  core  con- 
sisting of  a  bundle  of  iron  wires  usually  slipped  inside  an  ebonite 
tube.  Outside  the  ebonite  tube  is  wound  a  coil  of  comparatively 
thick  wire.  Outside  of  the  thick  wire  is  placed  a  good  thickness 
of  insulating  material,  depending  upon  the  pressure  it  is  desired 
to  create,  and  outside  of  that  a  long  length  of  fine  wire  is  wound. 
The  thick  wire  is  known  as  the  primary,  and  the  thin  wire  as  the 
secondary.  A  contact  breaker  completes  the  arrangement,  and  a 
battery  is  connected  to  the  primary  coil  through  the  contact  breaker. 
When  the  current  first  passes  in  the  primary  coil,  a  current  is 
generated  in  the  secondary  coil,  whose  pressure  is  approximately 
as  many  times  that  of  the  battery  furnishing  the  current  as  the 
number  of  turns  of  the  secondary  coil  are  greater  than  the  number  of 
turns  of  the  primary  coil.  When  the  current  passing  in  the  primary 
coil  is  broken  by  the  action  of  the  contact  breaker,  another  current  is 
furnished  in  the  secondary  coil  of  a  pressure  similar  to  the  first 
current,  but  in  the  opposite  direction,  the  first  current  being  in  the 
opposite  direction  to  that  of  the  primary  current,  and  the  second 
current  being  in  the  same  direction. 

As  in  all  these  matters,  it  is  not  necessary  that  the  current  shall 
be  actually  broken  for  inductive  action  to  take  place  between  the  two 
coils, — if  the  primary  coil  is  connected  to  a  source  of  electricity,  and 
any  variation  occurs  in  the  strength  of  the  primary  coil,  it  is  reflected 
in  the  secondary  coil  by  the  generation  of  a  current  either  in  opposi- 
tion to  that  of  the  primary  coil,  or  in  the  same  direction,  the  pressure 
in  each  case  depending  upon  the  arrangement  of  the  magnetic  circuit 
of  the  induction  coil,  and  upon  the  ratio  between  the  number  of  turns 
in  the  primary  and  in  the  secondary  coils.  Hence,  it  was  a  natural 
development  of  the  induction  coil,  though  it  only  occurred  to  a  few 
pioneers,  when  the  alternating  current  had  established  itself  for  the 
delivery  of  a  current  at  a  distance  for  electric  lighting,  that  the 
induction  coil  should  be  utilized  for  the  purpose  of  delivering  currents 
at  high  pressure,  and  using  them  at  low  pressure.  But  the  induction 
coil,  as  used  in  medical  and  X-ray  work,  shocking  coils,  etc.,  is  a  very 
inefficient  apparatus,  for  the  reason  that  no  attempt  has  been  made 
in  it  to  lower  the  resistance  of  the  magnetic  circuit.  Lines  of  force 
issue  from  the  ends  of  the  iron  core,  and  they  form  closed  curves 
passing  on  both  sides  through  the  air  between  the  ends  of  the  core, 
but  the  length  of  the  air  path  being  so  great,  the  resistance  offered  to 
the  passage  of  the  lines  of  force  is  very  high  indeed.  In  the  practical 
modern  transformer  this  defect  is  made  good. 


202  ELECTRICITY   IN   MINING 

Transformers  are  constructed  on  two  principal  main  lines,  called 
respectively,  core  transformers  and  shell  transformers.  In  both  forms 
the  iron  forms  a  closed  circuit,  and  the  iron  core  is  built  up  of  a 
number  of  thin  sheet  iron  plates,  insulated  from  each  other  very 
much  in  the  same  manner  as  the  plates  of  the  armature  of  a  dynamo. 
The  difference  between  the  construction  of  core  transformers  and  shell 
transformers  is,  in  the  core  transformer  the  primary  and  secondary 
coils  are  slipped  over  the  iron  cores,  in  the  shell  transformer  the  core 
surrounds  a  large  portion  of  the  coils.  In  the  core  transformer  the 
thin  iron  plates  are  built  into  a  framework  in  which  there  are  two  or 
three  legs,  according  as  the  transformer  is  for  single,  two,  or  three 
phase  currents.  The  legs  which  form  the  cores  inside  the  coils  are 
held  between  terminal  frames,  and  it  is  arranged  that  the  upper 
terminal  frame  can  be  removed,  the  wire  coils  slipped  over  the  core 
legs,  the  terminal  frame  replaced,  bolted  into  position,  and  the  whole 
thing  then  forms  the  closed  magnetic  circuit  described.  In  the 
shell  transformer  the  coils  are  made  in  the  form  of  large  open  rings 
nearly  rectangular  in  section,  the  iron  core  plates  being  then  fixed  so 
as  to  surround  the  coil  legs. 

The  operation  of  the  transformer  is  precisely  the  same  in  the  two 
forms,  the  object  of  the  construction  in  each  case  being  to  rechice  the 
magnetic  resistance,  and  thereby  to  reduce  the  losses  in  the  iron  and 
in  the  copper.  In  the  core  form  of  transformer  the  coils  in  which 
the  pressures  are  lowest  are  held  nearest  the  iron,  those  in  which  the 
pressures  are  highest  being  outside,  and  there  being  substantial 
insulation  between  the  two.  The  insulation  between  the  primary 
and  secondary  coils  consists  usually  of  a  cylinder  of  asbestos  or  some 
similar  material.  The  inside  coil  is  insulated  from  the  iron  as  well  as 
from  the  outside  coil.  Transformers  are  employed  for  raising  the 
pressure  as  well  as  for  lowering  it,  and  in  that  case  the  primary  coils 
would  be  those  in  which  the  pressure  is  lowest.  In  the  shell  type 
of  transformer,  the  primary  and  secondary  coils  are  made  in  sections, 
and  are  interleaved;  that  is  to  say,  the  secondary  coils  are  dis- 
tributed between  the  primary  coils,  so  that  the  induction  shall  be  also 
distributed  evenly  over  all  the  coils.  The  complete  mass  of  primary 
and  secondary  coils  is  carefully  insulated  from  the  iron  cores  which 
surround  them,  in  very  much  the  same  manner  as  armature  coils  and 
fielil  magnet  coils  are  insulated  from  their  iron  supports,  as  has  been 
explained. 

The  operation  of  the  transformers  is  as  follow.  Taking  first  a 
single-phase  alternating  current.  As  the  strength  of  the  current 
rises  in  the  primary  coil,  the  strength  of  the  magnetic  field  also 
rises,  and  a  pressure  is  created  in  the  secondary  coils  proportional 
to  the  rate  of  rise  in  the  strength  of  the  magnetic  field.  After 
the  primary  current  has  passed  its  maximum,  the  strength  of  the 


THE   GENERATION   OF   ELECTRICITY  203 

magnetic  field  commences  to  fall,  and  the  pressure  in  the  secondary 
coils  also  falls  till  the  second  zero  is  reached,  the  rise  of  current 
on  the  negative  side  in  the  primary  coils  being  followed  by  a  rise  of 
pressure  in  the  secondary  coils  to  the  maximum,  followed  by  a  fall  of 
pressure,  and  so  on. 

It  was  explained  in  Chapter  I.,  under  the  head  of  Self-induction 
and  the  Power  Factor,  that,  owing  to  the  induction  which  takes 
place  when  a  pressure  is  changing  in  strength,  as  an  alternating 
pressure  is,  the  current  which  results  from  the  application  of  a 
pressure  to  a  given  conductor  follows  the  pressure  a  certain  time, 
a  certain  fraction  of  the  whole  period  of  the  alternating  cycle,  after 
the  pressure,  and  this  happens  in  the  case  of  the  stationary  trans- 
former. The  current  in  the  primary  coils  follows  the  application 
of  the  pressure  a  little  after  the  pressure,  and  consequently  the 
pressure  in  the  secondary  is  also  a  little  after  the  pressure  in  the 
primary.  But  this,  again,  depends  entirely  upon  the  construction  of 
the  transformer,  and  upon  the  amount  of  work  it  is  doing.  With 
modern  transformers  it  is  claimed  that  when  the  transformer  is 
loaded  up  to  its  full  capacity  of  transformation,  the  power  factor  (see 
Chapter  I.,  p.  20)  is  as  high  as  0'99 ;  that  is  to  say,  the  current  lags 
only  a  very  short  period  indeed  behind  the  pressure,  and  there  is  only 
a  loss  of  1  per  cent,  in  the  transformation.  When  the  transformer,  how- 
ever, is  very  lightly  loaded,  as  where  it  is  used  for  lighting  purposes, 
and  during  the  major  portion  of  the  day  very  little  current  is  being 
taken  from  it,  the  power  factor  may  be  as  low  as  0'5  ;  that  is  to  say, 
the  current  in  this  case  will  lag  very  considerably  behind  the  pressure, 
and  there  will  be  a  practical  loss  of  50  per  cent,  in  transformation. 
Losses  in  transformation  due  to  the  power  factor  are  increased  by  the 
presence  on  the  secondary  circuit  of  inductive  apparatus,  such  as  induc- 
tion motors,  whose  construction  will  be  explained  in  Chapter  VI.  The 
matter  of  the  lag  of  the  secondary  pressure  behind  the  primary  makes 
no  difference  whatever  either  to  the  lighting  service  where  lights  are 
taken  from  the  current,  or  to  the  power  service ;  it  only  makes  a  differ- 
ence in  the  power  absorbed  by  the  generator  for  any  given  work. 

The  stationary  transformer  is  self-regulating.  It  calls  up  from  the 
primary  sendee  as  much  current  as  it  requires  to  furnish  the  current 
demanded  by  the  lamps  or  motors  on  the  secondary  service,  and  when 
lamps  or  motors  are  switched  off  on  the  secondary  service,  the  second- 
ary coils  cut  off  the  supply  that  is  no  longer  necessary  from  the 
primary  service.  And  the  self-regulation  arises  from  the  same  induc- 
tive action  which  causes  the  transformation  itself.  It  was  explained 
in  Chapter  I.  that  induction  took  place  between  wires  forming  parts 
of  different  circuits  when  currents  rose  or  fell  in  either  of  them,  and 
that  induction  also  took  place  in  any  circuit  when  the  wire  of  the 
circuit  was  coiled  on  itself,  as  in  an  electro-magnet,  when  changes  took 


204  ELECTRICITY   IN   MINING 

place  in  the  current  passing  in  the  circuit.  This  is  the  principle,  it 
will  be  remembered,  upon  which  the  choking  coil  used  with  alter- 
nating current  arc  lamps  is  worked.  In  the  stationary  transformer  it 
leads  to  self-regulation  in  the  following  manner.  When  the  secondary 
circuit  is 'open,  no  current  passing  through  its  coils,  the  primary  coils 
act  exactly  in  the  same  manner  as  the  choking  coil  of  an  arc  lamp. 
They  choke  back  the  current  that  is  not  wanted,  only  allowing  the 
minimum  current  to  pass  that  will  create  a  magnetic  field  in  the  core 
of  the  transformer,  sufficiently  powerful  to  respond  quickly  when  the 
secondary  circuit  calls  for  current.  When  currents  pass  in  the 
secondary  coils,  it  will  be  remembered  that  they  are  in  the  opposite 
direction  to  those  in  the  primary  coils,  with  the  result  that  as  the 
currents  passing  in  the  secondary  coils  are  increased,  the  choking 
action  in  the  primary  coils  is  lessened,  and  more  and  more  current 
passes  in  a  properly  designed  transformer,  exactly  in  proportion  to 
the  requirements  of  the  service. 

There  are  two  sources  of  loss  in  the  stationary  transformer,  known 
respectively  as  iron  loss  and  copper  loss.  The  copper  loss  is  that  due 
to  the  transformation  of  the  current  passing  through  the  coils  of  both 
primary  and  secondary  into  heat.  It  is  known  as  the  C2E  loss,  and  it 
is  measured  in  both  primary  and  secondary  coils  by  the  square  of  the 
current  in  each  multiplied  by  the  resistance  of  each.  The  iron  loss 
is  that  due  to  the  heating  of  the  iron  core  by  what  is  known  as 
hysteresis.  It  was  explained  in  Chapter  I.  that  on  Professor  Swing's 
theory  of  magnetism,  the  molecules  of  the  iron  which  are  being 
magnetized  swing  round  on  their  axes,  swinging  back  when  the 
impelling  force  ceases,  swinging  in  the  opposite  direction,  in  obedience 
to  the  impelling  magetizing  force  in  the  opposite  direction,  and  so  on. 
In  the  cores  of  transformers,  the  iron  molecules  are  subject  to 
magnetizing  forces  in  opposite  directions,  changing  twice  during 
every  cycle  of  the  current;  hence  assuming  Swing's  theory  to  be 
correct,  the  molecules  are  subject  to  a  continued  swinging  to  and  fro 
during  each  cycle,  and  this  results  in  the  liberation  of  a  certain 
quantity  of  heat  in  the  iron  itself.  The  energy  delivered  to  the  iron 
core  in  the  form  of  heat  in  this  manner,  must  be  taken  from  the 
current  supplying  the  transformer  with  energy,  and  is  therefore  a 
source  of  loss.  Modern  transformers  are  designed  so  that  at  full  load 
the  iron  losses  and  the  copper  losses  are  equal,  the  sum  of  the  two 
being  only  the  small  fraction  mentioned  above,  1  per  cent,  of  the 
total  power  dealt  with  by  the  transformer. 


THE   GENERATION   OF   ELECTRICITY  205 


Transformers  for  Single,  Two,  and  Three 
Phase  Currents 

The  construction  of  transformers  for  single  and  two  phase  currents 
is  practically  the  same.  With  the  core-type  transformers  there  are 
two  legs,  carrying  the  coils  as  described,  and  the  coils  are  frequently 
arranged  so  that  the  transformer  can  be  employed  on  services  for  100 
or  110,  and  for  200  or  220  volts,  the  coils  on  the  two  legs  being  con- 
nected in  parallel  for  the  lower  pressure,  and  in  series  for  the  higher 
pressure.  For  two-phase  services,  exactly  the  same  construction  is 
employed ;  there  are  two  legs,  but  each  leg  is  connected  to  one  of  the 
phases.  With  three-phase  services  there  are  three  legs,  each  having 
its  own  coils,  and  each  set  of  coils  being  connected  to  one  phase.  For 
a  larger  number  of  phases  there  would  be  more  legs  in  proportion. 
It  will  be  understood  that  in  the  construction  for  the  single-phase 
transformer  two  legs  are  necessary,  because  a  complete  magnetic  circuit 
cannot  be  formed  in  any  other  way.  With  shell  transformers  the 
arrangement,  for  single,  two,  and  three  phase,  is  made  in  various 
ways,  but  the  main  lines,  as  described  above,  are  followed.  There  is 
practically  no  difference  between  the  construction  of  a  single  and  a 
two  phase  shell  transformer,  except  that  there  are  separate  sets  of 
coils  with  two-phase,  while  there  is  only  one  with  single-phase ;  and 
by  a  simple  extension,  three-phase  have  three  separate  sets  of  coils. 


Cooling  Stationary  Transformers 

It  was  mentioned  above  that  the  coils  and  the  iron  core  both  had 
a  certain  quantity  of  heat  liberated  in  them,  and  it  is  important  that 
the  heat  shall  be  dissipated  as  it  is  formed  for  two  reasons.  As  was 
explained  in  Chapter  I.,  all  metals  increase  their  resistance  with 
increase  of  temperature,  and  in  a  definite  proportion  to  the  increase ; 
therefore,  as  the  temperature  of  the  copper  coils  of  a  transformer 
increases,  their  resistance  also  increasing,  the  current  passing  through 
them  decreases,  and  according  to  the  formula  H  =  C2E,  the  loss  in 
the  coils,  with  a  given  current,  also  increases.  In  the  case  of  the 
heating  of  the  iron  another  curious  result  was  discovered  by  Mr. 
Mordey,  and  confirmed  by  other  observers,  some  few  years  since,  viz. 
that  if  the  iron  cores  on  transformers  are  allowed  to  be  raised  to  a 
certain  temperature,  what  has  been  termed  "ageing"  takes  place. 
The  iron  increases  its  magnetic  resistance,  the  output  of  any  given 
transformer  being  thereby  decreased.  Both  of  these  troubles  are  got 
rid  of  by  not  allowing  the  transformers  to  increase  their  temperature 


206  ELECTRICITY   IN    MINING 

beyond  a  certain  figure.  In  the  best  modern  transformers  the 
increase  of  temperature  does  not  exceed  45°  C.  =  113°  Fahr.,  and  this 
result  is  accomplished  partly  in  the  construction  of  the  transformers, 
by  allowing  plenty  of  iron  and  plenty  of  copper,  by  using  only 
special  brands  of  iron  or  steel,  specially  annealed  for  the  purpose,  and 
only  pure  copper,  and  being  very  particular  about  the  insulation  of 
the  copper ;  and  partly  by  arranging  to  dissipate  a  portion  of  the  heat 
liberated.  There  are  three  methods  of  dissipating  the  heat  liberated 
in  a  transformer — by  air,  by  oil,  and  by  water.  In  this  country 
water  has  not  been  employed,  but  it  has  been  in  America.  With  air- 
cooled  transformers,  the  transformer,  constructed  as  described,  is  fixed 
inside  an  iron  case,  which  is  provided  on  its  outside  with  ribs  or  cor- 
rugations intended  to  increase  the  surface  in  contact  with  the  air,  and 
the  whole  apparatus  is  fixed  either  in  some  position  where  there  is  a 
natural  draught  sufficient  for  the  purpose,  or  a  current  of  air  is  passed 
over  it  by  the  aid  of  a  fan.  The  oil-cooled  transformer  is  fixed  inside 
a  tank  which  is  filled  with  oil  whose  flash-point  is  about  350°  Fahr., 
the  oil  performing  the  double  office  of  dissipating  and  equalizing  the 
heat,  and  sealing  up  any  defects  in  the  insulation  that  may  arise.  In 
America  the  Allis-Chalmers  Co.  construct  modifications  of  the  oil- 
cooled  transformer,  in  some  of  which  the  tanks  are  fitted  with  ribs,  so 
that  the  air  takes  a  part  in  the  cooling,  and  in  others  there  is  a  coil  of 
pipe  enclosed  in  the  transformer  chamber,  through  which  water  is 
circulated,  the  office  of  the  water  being  to  carry  off  heat  from  the  oil, 
the  oil  in  its  turn  receiving  the  heat  from  the  transformer  coils  and 
the  ironwork.  If  the  matter  is  worth  the  additional  complication,  and 
where  there  is  a  cold-storage  plant  on  the  ground,  as  possibly  many 
collieries  will  have  before  many  years  are  over,  especially  those 
having  high  temperatures  at  great  depths,  cold  brine  at  any  desired 
temperature  could  be  circulated  through  a  pipe  enclosed  in  the 
transformer  chamber,  and  any  quantity  of  heat  could  be  carried  off 
thereby.  Further,  at  those  collieries  where  compressed  air  is  in  use, 
a  blast  from  a  compressed  air  plant  should  have  a  very  cooling  effect 
upon  transformers. 


Arrangement  of  Apparatus  in  the  Generating 

Station 

For  the  economical  working  of  any  electricity-generating  station, 
certain  conditions  have  to  be  complied  with.  The  generating  appa- 
ratus are  usually  divided  into  units,  and,  where  it  can  be  so  arranged, 
each  unit  with  its  accessories  is  complete  in  itself.  Thus  a  unit  will 
consist  of  a  generator,  with  its  excitor  where  alternating  currents  are 
employed,  its  driving  engine,  whether  this  be  steam,  gas,  oil,  or  water, 


THE   GENERATION   OF  ELECTRICITY  207 

together  with  sufficient  apparatus  to  provide  the  gas  or  steam  engine 
with  sufficient  gas  or  steam  to  do  the  full  work  the  unit  is  intended 
to  deal  with.  Thus,  with  steam  engines,  each  generator  will  have  its 
engine,  its  bank  of  boilers  sufficient  to  supply  steam  for  the  full 
output  of  the  generator  its  condensers  with  its  circulating  and  air 
pumps,  its  feed  pumps,  etc.  With  gas  engines  the  engine  will  have 
its  own  gas  producer  and  scrubber,  etc.,  which  will  be  of  sufficient  size 
to  provide  gas  for  the  full  output  of  the  generator.  There  are,  of 
course,  variations  of  these  arrangements.  The  whole  of  the  con- 
densing of  any  station,  and,  in  later  practice,  of  a  group  of  stations, 
may  be  performed  at  one  condensing  station.  The  whole  of  the  feed- 
water  may  be  dealt  with  in  one  set  of  feed-water  heaters  and 
economizers,  and  so  on.  Plate  9B  shows  a  generating  station  run  by 
Pelton  water  wheels.  Plate  10  shows  a  generating  station  driven  by 
Oechelhausen  gas  engines.  Plate  11  shows  a  complete  suction  gas 
engine  plant. 

The  Size  of  the  Units 

The  size  of  the  unit  is  naturally  of  considerable  importance,  and 
it  is  governed  by  two  considerations.  The  unit,  or  a  multiple  of  the 
unit,  preferably  the  unit  itself,  must  be  sufficiently  large  to  deal  with 
the  whole  of  the  load  during  times  of  light  load.  Practically  the 
extent  of  the  light  load  fixes  the  size  of  the  unit,  though  where  there 
is  a  very  considerable  difference  between  the  light  load  and  full  load 
it  is  sometimes  arranged  that  the  light  load  is  dealt  with  by  a  small 
unit,  and  the  full  load  by  one  or  more  larger  units.  The  best  plan, 
in  the  author's  opinion,  is  to  make  the  size  of  the  unit  that  which 
will  deal  with  the  light  load,  and  to  multiply  the  units  to  whatever 
extent  may  be  required  to  deal  with  the  full  load.  The  other  con- 
sideration upon  which  the  size  of  the  unit  depends,  is  the  question  of 
the  efficiency  of  the  plant  as  a  whole,  and  the  cost  of  the  plant.  The 
cost  of  very  large  units  is  very  much  less  per  kilowatt  than  of 
smaller  units.  Also,  large  units  are  usually  more  efficient,  they 
make  a  smaller  charge  for  converting  mechanical  to  electrical  energy 
than  the  smaller  units.  Between,  however,  the  size  of  the  unit  that 
will  deal  comfortably  with  the  whole  of  the  light  load,  and  that  which 
would  deal  with  the  full  load,  in  a  smaller  number  of  units  than 
would  be  required  when  making  the  light  load  the  unit,  there  is  not 
very  much  difference  either  in  the  cost  per  kilowatt  of  the  machinery, 
or  in  the  efficiency.  In  any  case,  the  latest  practice  is,  a  certain 
number  of  the  units  decided  upon  are  fixed,  the  generators  and  their 
driving  accessories  in  one  building,  the  boilers  and  their  accessories  in 
an  adjoining  building,  the  total  number  of  the  units  being  sufficient  to 
deal  with  the  whole  of  the  load  the  generating  station  may  be 


208  ELECTRICITY   IN   MINING 

required  to  supply,  plus  at  least  one  unit  as  a  stand-by  against  the 
breakdown  of  any  of  the  others.  It  then  becomes  a  very  simple 
matter,  as  will  be  explained  in  Chapter  V.,  to  proportion  the  number 
of  units  in  service  to  the  load,  and  to  remove  any  unit  from  service 
that  shows  signs  of  failing,  and  to  replace  it  by  a  spare  unit  in  a  very 
short  time.  There  is  another  question  that  deserves  consideration, 
especially  in  connection  with  mining  work,  and  that  is  how  far  each 
individual  unit  should  be  worked  up  to  its  full  output.  The  dif- 
ference usually,  between  the  efficiency  of  units  working  at  full  load 
and  at  three-quarter  load,  is  not  great.  The  practice  in  some  gene- 
rating stations  is  to  work  all  units  in  the  service  at  their  full  load, 
but  this  has  the  disadvantage  that  there  is  no  margin  in  case  of  a 
sudden  call.  The  author  would  prefer,  for  mining  work  at  any  rate, 
that  units  should  be  worked  at  something  like  three-quarters  their 
possible  load,  or  even  less.  As  all  modern  generators  are  constructed 
to  work  at  twenty-five  per  cent,  overload  for  an  hour,  without  danger, 
this  means  that  in  case  of  accidents,  such  as  unfortunately  happen 
occasionally,  where  two  or  more  generators  break  down  together,  there 
is  a  certain  margin  in  each  unit  to  meet  the  breakdown,  if  they  are 
worked  under  their  full  possible  load. 


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CHAPTER  V 
DISTRIBUTION  OF  POWER  BY  ELECTRICITY 

IT  was  explained  in  Chapter  I.  that  the  power  in  any  electric  circuit  is 
measured  by  the  product  of  the  two  factors,  the  pressure  and  the 
current  passing  in  the  circuit,  with  the  qualification,  in  the  case  of 
alternating  currents,  that  the  difference  in  time  between  the  current 
and  the  pressure  which  creates  it  should  be  taken  account  of  in  the 
power  factor.  It  will,  perhaps,  be  as  well  to  explain  that  this  rule 
holds  good  for  every  case  where  power  enters.  Thus,  the  electrical 
energy  taken  by  an  incandescent  lamp,  by  an  arc  lamp,  or  by  a  motor, 
is  measured  by  the  product  of  the  pressure  at  the  terminals  of  the 
lamp  or  motor  while  the  current  is  passing,  multiplied  by  the  current 
passing  through  the  lamp  or  motor  at  any  instant,  the  product  being 
multiplied  by  the  power  factor  where  the  current  is  alternating ;  and 
the  electrical  energy  may  at  all  times  be  converted  into  the  equivalent 
mechanical  energy  by  dividing  the  product  by  746,  this  being  the 
number  of  watts  per  horse-power ;  that  is  to  say,  the  rate  at  which 
energy  is  being  expended  in  lamps  or  motors  or  cables,  is  measured  as 
above.  Further,  it  should  be  mentioned  that  every  apparatus  that  is 
engaged  in  transmitting  the  energy,  or  converting  it  from  one  form 
to  another,  takes  toll  of  the  energy.  Thus  the  energy,  in  the  form 
of  heat  that  is  delivered  to  the  water  in  the  steam  boiler,  is  not 
the  whole  of  the  energy  liberated  by  the  combustion  of  the  coal  in  the 
boiler  furnace.  Again,  the  energy  delivered  at  the  crankshaft  of  the 
steam  engine  or  the  shaft  of  the  steam  turbine  is  not  the  whole  of 
the  energy  either  that  was  delivered  to  the  water,  or  that  was  present 
in  the  steam  which  entered  the  steam  cylinder  or  turbine  chamber. 
Again,  the  mechanical  energy  delivered  by  the  steam  motor  to  the 
electric  generator  does  not  all  appear  as  electrical  energy  at  the 
terminals  of  the  generator.  The  generator  makes  a  charge  varying 
from  seven  per  cent,  to  twenty  per  cent,  for  converting  the  mechanical 
to  electrical  energy.  The  cables  through  which  the  currents  pass  to 
the  lamps  or  motors,  again,  make  their  charge  for  transmitting  the 

209  p 


2io  ELECTRICITY  IN  MINING 

current,  the  charge  being  measured  in  this  case  by  the  product  of  the 
fall  of  pressure  between  the  terminals  of  the  generator  and  the  ter- 
minals of  the  lamp,  motor,  distributing  point,  etc.,  multiplied  by  the 
current  passing  through  the  cables.  Thus,  if  current  is  delivered 
at  the  terminals  of  the  generator  at  a  pressure  of  550  volts,  and  the 
pressure  at  a  distributing  point  within  the  mine,  say,  at  the  pit 
bottom,  or  at  a  point  farther  in-bye,  is  only  500  volts,  the  cables  will 
have  made  a  charge  upon  the  pressure  of  50  volts  for  transmitting 
the  power  through  them.  If  the  current  transmitted  is  100  amps., 
the  total  charge  made  by  the  cables  is  5000  watts,  or  about  6*6  H.P. 
Further,  it  should  be  understood  that  there  is  a  continual  fall  of 
pressure  from  the  terminals  of  the  generator  outwards,  whenever  any 
current  is  passing,  and  that  the  fall  will  be  proportional  to  the  product 
of  the  current  passing,  into  the  resistance  of  the  cables,  or  other  con- 
ductors through  which  it  passes.  Thus  there  will  be  a  small  fall  of 
pressure  between  the  terminals  of  the  generator  and  the  switchboard, 
to  be  presently  described.  There  will  be  a  further  fall  of  pressure 
between  the  main  switchboard  in  the  generator  house  and  the  switch- 
board or  distributing  arrangement  at  the  pit  bottom,  and  a  further 
fall  of  pressure  between  the  pit  bottom  and  any  distributing  point, 
say,  one  near  the  face  for  the  supply  of  coal-cutting  machines.  There 
is  a  continual  fall  of  pressure  through  any  apparatus  that  is  using  the 
current,  and  always  in  the  same  proportion,  bearing  in  mind  that  the 
current  and  pressure  to  be  employed  in  calculations  of  this  kind  for 
alternating  currents  are  the  virtual  or  effective  volts  and  amperes. 
Further,  it  is  well  to  note  that  the  fall  in  pressure  at  any  point,  such 
as  the  distributing  point  mentioned,  or  the  terminals  of  a  lamp  or 
motor,  vary  directly  in  accordance  with  the  formula,  E  =  CE,  E  being 
the  fall  in  pressure,  C  the  current  passing,  and  K  the  resistance  between 
the  terminals  of  the  generator,  or  any  other  point  taken  as  a  starting- 
point,  such  as  the  main  switchboard  and  the  terminals  of  the  appa- 
ratus. This  is  a  very  important  matter  in  connection  with  the  use 
of  electrical  apparatus,  and  the  size  of  the  cables  required.  Incan- 
descent lamps,  for  instance,  which  are  supplied  from  cables  that  are 
also  supplying  motors,  or  other  current  users,  will  have  their  light 
increased  or  decreased,  according  as  the  other  apparatus  are  taking 
less  or  more  current,  and  in  the  case  of  arc  lamps  it  is  sometimes 
difficult  to  keep  them  working  if  the  pressure  varies  much  at  their 
terminals.  To  take  an  instance.  Suppose  a  pair  of  cables  for  con- 
tinuous current  to  be  fixed  between  the  main  switchboard  and  the  pit 
bottom,  and  to  have  a  resistance  of  0*1  ohm,  and  suppose  the  pressure 
at  the  switchboard  to  be  550  volts.  Suppose,  also,  that  a  group  of 
lamps  of  any  kind  are  taking  current  from  these  cables  in  the  neigh- 
bourhood of  the  pit  bottom,  and  that  a  haulage  motor  is  also  taking 
current  from  it,  and  that  the  haulage  motor  when  at  work  takes  a 


DISTRIBUTION   OF  POWER   BY  ELECTRICITY    211 

current  from  100  to  400  amps.  When  the  haulage  motor  is  standing, 
supposing,  for  the  moment,  that  no  other  apparatus  is  taking  current 
except  the  lamps,  and  that  they  are  only  taking  a  small  current,  the 
pressure  at  the  terminals  of  the  series  of  lamps  will  be  550  volts,  or 
thereabouts.  When  the  haulage  motor  is  started,  and  is  taking  its 
400  amps.,  the  pressure  at  the  terminals  of  the  cables  at  the  pit  bottom 
will  drop  to  510  volts,  and  will  then,  as  the  motor  gets  hold  of  its 
load,  rise  to  520,  530,  and  540  volts.  This  means  that  the  pressure 
at  the  terminals  of  the  series  of  lamps,  and  at  those  of  each  individual 
lamp,  will  fall  and  rise  in  the  same  ratio.  With  the  ordinary  carbon 
incandescent  lamp  this  only  means  that  the  light  will  vary,  and  pro- 
bably the  lamp  filament  will  not  last  as  long  as  if  the  pressure  was 
always  constant.  With  arc  lamps  or  Nernst  lamps  it  may  mean  that 
the  lamps  will  go  out,  unless  special  provision  has  been  made  to  com- 
pensate for  these  variations. 

The  author  has  thought  it  wise,  in  commencing  this  chapter, 
to  go  rather  fully  into  this  question,  because  it  is  of  such  great 
importance,  in  the  matter  of  electrical  distribution,  to  bear  in  mind 
that  pressures  vary  in  the  manner  described.  He  has  also  illus- 
trated the  matter  by  reference  to  continuous  currents  because  it  is 
simpler ;  but  the  same  thing  rules  with  alternating  currents,  bearing 
in  mind  what  has  been  explained  about  the  pressures  to  be  used, 
and  also,  as  will  be  explained  in  Chapter  VI.,  that  motors,  when 
they  start,  take  large  currents,  and  when  they  have  got  hold  of 
their  load,  take  comparatively  smaller  currents,  and  so  on. 


Conductors  for  Electric  Light  and   Power 
Distribution 

The  conductors  for  distributing  current  for  light  and  power  are 
almost  universally  of  copper.  Iron  has  been  employed  to  a  very 
small  extent  in  the  form  of  old  wire  ropes  by  Mr.  Arthur  Sopwith 
and  others,  in  the  early  days  of  electric  lighting  in  mines,  but  though 
Mr.  Sopwith  achieved  very  good  results,  it  is  hardly  a  satisfactory 
arrangement.  In  the  first  place,  iron  and  steel  have  from  six  to 
seven  times  the  electrical  resistance  of  copper,  which  means  that  the 
conductors  employed  have  to  be  six  or  seven  times  as  heavy,  and  as 
large  in  sectional  area ;  and,  in  addition  to  this,  old  iron  or  steel  wire 
ropes  have  to  be  very  much  heavier  and  larger  in  every  way  than  an 
iron  conductor  would  be  under  ordinary  circumstances,  because  the 
old  rope  is  really  a  mass  of  broken  iron  wires  held  together  by  the 
construction  of  the  rope,  in  which  conduction  takes  place  very  often 
between  the  surfaces  of  the  broken  wires,  the  wires  themselves  being 
oxidized  or  covered  with  grease.  Hence,  when  large  currents  come 


212  ELECTRICITY  IN   MINING 

to  be  dealt  with,  a  limit  is  soon  reached,  beyond  which,  if  it  were 
advisable  for  other  reasons  to  employ  old  wire  ropes,  their  size 
becomes  utterly  unmanageable.  Aluminium  is  also  gradually 
coming  to  the  front,  but  it  is  hardly  yet  in  that  form  in  which  it 
could  be  recommended  for  mining  work.  When  it  is  in  a  really 
practical  condition  it  will  be  cheaper  than  copper,  and  probably,  as 
the  development  of  its  manufacture  increases,  it  will  continue  to 
become  cheaper.  The  specific  gravity  of  aluminium  is  less  than 
one-third  that  of  copper,  while  its  conductivity  is  about  as  3  to  5 
compared  with  copper.  Its  price  ranges  from  three  to  four  times 
that  of  copper,  according  to  the  price  of  the  latter  at  the  moment, 
so  that  on  the  whole,  taking  all  points  into  consideration,  it  would 
be  cheaper  even  at  its  present  price;  but  there  is  an  element  of 
uncertainty  about  it  still.  The  metal  itself  is  not  yet  thoroughly 
understood.  Aluminium  has  properties  quite  different  to  those  of 
other  metals.  The  question  of  jointing  is  a  very  troublesome  one. 
So  far  as  the  author  is  aware,  there  is  no  aluminium  joint  on  the 
market  that  can  be  thoroughly  recommended  for  use  about  mines. 
The  best  form  of  joint  he  knows  of  consists  of  a  sleeve,  which  is 
slipped  over  the  two  ends  of  the  wire  to  be  jointed,  and  squeezed 
down  upon  both  of  them.  Soldering  does  not  appear  to  be  prac- 
ticable. There  is  also  an  element  of  uncertainty  as  to  the  behaviour 
of  aluminium  in  the  open,  in  the  presence  of  the  atmosphere,  and 
any  gases  there  may  be  in  it.  In  America  several  attempts  have 
been  made  to  use  aluminium  conductors  for  overhead  cross-country 
transmission  lines,  with  varying  success. 


Overhead  Conductors 

Where  a  group  of  mines  are  supplied  with  current  from  a  single 
generating  station,  it  is  perfectly  practicable  and  legitimate  to 
employ  naked  copper  conductors,  carried  overhead  between  the 
generating  station  and  the  principal  distributing  points,  such  as  the 
sub-stations  at  the  different  mines.  The  copper  conductors  that  are 
employed  for  this  work  are  sometimes  solid,  but  more  frequently 
stranded.  The  limit  to  the  size  of  a  solid  copper  conductor  that  can 
be  handled  comfortably  is  about  0*5  of  an  inch  in  diameter.  For  the 
same  reason  that  insulated  copper  conductors  are  stranded  after  a 
certain  size  is  reached,  overhead  conductors  are  also  more  con- 
veniently stranded.  A  stranded  conductor  is  very  much  more 
flexible,  and  more  easily  handled  in  every  way,  than  a  solid  con- 
ductor of  the  same  sectional  area.  On  the  other  hand,  a  stranded 
conductor  presents  a  very  much  larger  surface  to  the  action  of  the 
oxygen  of  the  atmosphere,  and  therefore  will  probably  not  last  as 


DISTRIBUTION   OF  POWER  BY   ELECTRICITY     213 

long  where  there  is  much  smoke,  and  where,  as  usually  happens 
with  smoke,  there  is  sulphuric  acid  in  the  atmosphere.  For  these 
reasons  solid  conductors  are  employed  in  manufacturing  districts, 
while  stranded  conductors  may  be  employed  in  country  districts. 
Overhead  conductors  are  supported  by  porcelain  insulators,  and  these 
are  now  usually  of  what  is  known  as  the  triple  petticoat  form  shown 


HIGH  TENSION  INSULATORS 


£11374-. 


£1  /&7/ 


FIG.  90.— Various  Forms  of  Triple  Petticoat  Insulators  made  by  Messrs. 
Buller.  It  will  be  noticed  tbat  in  all  of  them  the  Surface  over  which 
the  Leakage  Current  must  pass  is  made  as  long  as  possible. 

in  Fig.  90.  This  form  of  insulator,  which  has  been  developed 
principally  in  America,  is  made  very  strong,  of  a  very  high  insulation 
resistance,  a  very  high  resistance  to  sparking  through  the  substance 
of  the  insulator ;  and  the  surface,  as  will  be  seen,  is  so  arranged  that 
any  leakage  current  has  a  very  long  path  to  pass  over  between  the 


214 


ELECTRICITY   IN   MINING 


conductor  and  the  bolt  supporting  the  insulator.  Considerable  dif- 
ference of  opinion  exists  among  American  engineers,  who  have  had 
the  largest  experience  in  overhead  work  of  this  kind,  as  to  the 
material  of  which  the  supporting  bolt  of  the  insulator  should  be 


FIG.  91. — Special  Form  of  High  Tension  Insulator  made  by  Messrs.  Buller. 
It  will  carry  Wires  either  in  the  Groove  on  the  Top  or  in  that  on 
the  Side. 

made.  Conditions  of  strength  point  to  iron  or  steel,  properly  galva- 
nized, as  the  best  material,  and  this  is  the  substance  employed  for 
the  insulator  bolts  of  telephone  and  telegraph  lines.  But  other  con- 
ditions in  America  have  led  to  the  adoption  of  creosoted  wooden 


DISTRIBUTION    OF   POWER   BY   ELECTRICITY     215 

insulator  bolts,  these  being  said  to  give  a  higher  insulation  resistance, 
and    to   stand   climatic  and  other  influences  better  than  iron.      In 
this  country  iron  or  steel  is  generally  employed,  and  the  insulator 
is  supported  by  its  bolt  upon  brackets  or  arms,  carried  by  poles, 
fixed  in  any  convenient  position.     The  poles  may  be  of  wood,  and 
should  then  be  creosoted,  the  insulators  being  supported  upon  creo- 
soted  arms  bolted  to  the  poles ;  or  the  poles  may  be  of  iron,  which, 
again,  may  consist  of  latticework,  similar  to  that  employed  in  some 
of  the  modern  pit  headstocks,  and  in  some  of  the  railway  signal 
posts ;  or  they  may  consist  simply  of  tubes  made  in  definite  lengths, 
and  arranged  to  fix  one  on  top  of  the  other,  the  smaller  one  in  each 
case  fitting  into  a  socket  in  the  top  of  the  larger  one.     It  is  best  also, 
where  conductors  cross  public  roads,  or  where  they  would  be  liable  to 
cause  damage  if  they  fall,  to  protect  them  by  guards  of  some  kind 
placed  under  the  wires.      There  are  several  forms  of  guards ;  one 
consists  of  an  iron  strap  bent  round  the  insulator  and  bolted  to  the 
pole  above  and  below,  so  that  if  the  conductor  leaves  its  insulator,  or 
if  it  becomes  slack,  in  place  of  falling,  it  is  supported  by  the  iron 
strap.     Another  arrangement  is,  iron  wires  are  stretched  between  the 
poles,  with  connecting  wires  crossing  them,  forming  a  kind  of  open 
trough  or  cradle.     If  the  conductors  come  away  from  their  insulators, 
they  are  caught  by  the  cradle.     There  is  another  danger  with  naked 
overhead  conductors  that  must  be  provided  for,  viz.  the  possibility 
of  mischievous  boys  climbing  the  poles  and  either  getting  shocks 
from  the  conductors,  or  placing  pieces  of  metal  between  the  con- 
ductors for  the  purpose  of  seeing  the  arc  that  is  formed.     It  is  not 
easy  to  provide  against  this,  but  possibly  the  best  method  is  a  sub- 
stantial wrapping  of  barbed  wire  for  a  certain  distance  above  the 
ground,  or  provision  of  spiked  rings,  something  after  the  pattern  of 
a  cheval   de  frise.     Probably  a  vigorous  application   of  the   police 
court  would  be  the  best  preventative.     It  is  also  necessary  that  any 
conductors  about  the  poles,  including  the  poles  themselves  if  they 
are  of  iron,  should  be  well  earthed.     In  some  cases  earthing  arms 
are  provided,  consisting  of  iron  arms  or  brackets  connected  to  earth, 
and  arranged  to  catch  a  falling  conductor,  and  therefore,  presumably, 
putting  the  conductor  to  earth,  and  rendering  it  dead  immediately  it 
touches  the  arm. 


Insulated  Conductors 

There  are  four  substances  employed  for  the  insulation  of  con- 
ductors— gutta  percha,  indiarubber,  bitumen,  and  yarn  fibre,  or  paper, 
the  latter  substances  being  impregnated  in  resinous  oils.  All  of  the 
insulating  substances  are  hydrocarbons,  and  they  will  all,  if  provided 


2i 6  ELECTRICITY  IN   MINING 

with  a  sufficient  quantity  of  heat,  become  gases,  and  will  then  behave 
very  much  in  the  same  manner  as  the  explosive  gas  given  off  in  a 
coal-mine.  If  a  light  be  brought  to  a  mixture  of  the  gas,  formed 
from  the  insulating  material,  and  air,  when  they  are  in  certain 
proportions,  an  explosion  will  follow.  This  has  been  the  cause  of 
several  of  the  explosions  that  have  taken  place  in  connection  with 
the  underground  conductors  in  the  streets  of  towns,  some  of  which 
have  been  erroneously  laid  at  the  door  of  the  illuminating  gas,  which 
is  carried  in  pipes  near  the  conductors.  Gutta  percha  is  hardly  ever 
used  for  conductors  for  light  and  power,  because  its  melting-point  is 
so  low,  and  it  softens  at  such  a  low  temperature  that  the  conductor 
is  easily  thrown  out  of  the  centre  of  the  insulating  envelope,  and  the 
insulation  on  that  side  becomes  very  much  reduced.  For  wires, 
however,  for  signals  and  telephones,  especially  where  wet  is  always 
present,  and  where  there  is  no  light,  as  in  mine  shafts,  gutta  percha 
is  the  very  best  material  that  can  be  employed.  For  small  con- 
ductors, also,  for  electric  lighting  work,  it  might  be  employed  with 
care  under  similar  conditions.  Gutta  percha  is  almost  indestructible 
under  water  when  protected  from  light.  As  with  so  many  other 
substances,  however,  the  cost  of  gutta  percha  has  increased  during 
the  last  twenty  years,  and  there  are  many  substitutes  on  the  market, 
containing  only  a  comparatively  small  proportion  of  gutta  percha, 
and  these  substances  have  not  the  properties  of  the  pure  material. 
It  should  be  noted  that  if  gutta  percha  is  employed  as  an  insulator, 
the  thickness  of  the  insulation  should  be  as  great  as  possible.  With 
very  thin  coatings  of  gutta  percha,  even  when  the  substance  is  pure, 
water,  such  as  is  found  in  most  pit  shafts,  will  find  its  way  through. 

Indiarubber  is  the  substance  that  bears  probably  the  best  name 
for  the  insulation  of  conductors  for  mining  work,  but  it  is  on  the 
condition  that  there  is  a  very  substantial  thickness  of  the  rubber 
outside  of  the  conductor,  that  the  rubber  is  properly  laid  on,  and 
that  it  consists  of  proper  materials.  Pure  rubber  does  not  stand 
wet,  and  it  is  acted  upon  by  copper,  therefore  it  is  usual,  where  the 
rubber  will  be  exposed  to  wet,  to  submit  it  to  a  process  known  as 
vulcanizing,  in  which  a  certain  quantity  of  suphur  is  mixed  with  the 
rubber,  the  compound  being  baked  at  a  certain  temperature  after  it 
has  been  laid  around  the  conductor.  In  practice,  rubber-covered 
cables  are  constructed  as  follows.  The  outer  conductors  of  the  strand 
are  tinned,  and  the  tinning  should  be  very  carefully  carried  out,  as 
where  this  is  not  done  the  copper  will  be  brought  into  contact  with 
the  rubber,  and  deterioration  will  set  in.  Next  to  the  conductors 
are  placed  two  wrappings  of  pure  rubber  strip,  laid  on  transversely 
and  in  opposite  directions,  so  that  the  joints  of  the  strips  cross. 
Outside  of  the  pure  rubber  is  placed  a  layer  of  what  is  known  as 
"  intermediate,"  consisting  of  rubber  to  which  has  been  added  a 


PLATE  12A. — Main  Switch  Board  for  Continuous  Currents,  made  by 
Messrs.  Reyrolle,  for  Mining  Work. 


PLATE  12s. — Back  of  Main  Switch  Board  shown  in  Plate  12 A,  made 
by  Messrs.  Reyrolle.  It  will  be  noticed  that  there  is  plenty  of 
room  for  a  man  to  work  without  danger. 

[To  face  p.  216. 


DISTRIBUTION   OF   POWER   BY  ELECTRICITY    217 

certain  quantity  of  a  salt  containing  sulphur,  such  as  the  sulphide 
of  antimony.  Outside  of  the  intermediate  is  placed  another 
coating  called  "jacket,"  consisting  of  vulcanized  rubber.  Inter- 
mediate and  jacket  are  placed  on  the  conductor  longitudinally 
in  two  strips,  which  are  pressed  round  the  conductor,  cut  off,  and 
jointed  by  one  machine.  The  conductor  that  is  being  insulated 
passes  through  the  different  machines  in  succession.  It  receives  one 
lap  of  rubber  in  one  machine,  a  second  lap  in  a  second  machine, 
intermediate  in  a  third  machine,  and  jacket  in  a  fourth,  emerg- 
ing from  the  last  machine  completely  insulated,  except  for  the 
baking  process  and  the  braiding,  etc.  After  the  rubber  has  been 
placed  on  the  cable,  the  coils  of  cable  are  placed  in  a  drum,  which 
is  heated  for  several  hours,  this  welding  the  three  coverings,  the 
outer  rubber,  the  intermediate,  and  the  jacket,  into  one  homo- 
generous  envelope  surrounding  the  conductor.  After  baking,  the 
cable  is  usually  taped,  and  sometimes  braided,  sometimes  armoured, 
sometimes  covered  with  lead,  sometimes  covered  with  lead  and  then 
armoured.  There  are  two  great  difficulties  about  the  employment 
of  rubber  insulation.  All  the  gums  of  which  rubber  and  the  other 
insulating  substances  are  composed,  oxidize  freely  if  exposed  to  air 
or  moisture,  and  this  leads  to  the  gradual  disintegration  of  the  rubber, 
and  the  gradual  penetration  of  the  moisture,  if  present,  to  the 
conductor,  the  insulation  being  destroyed  in  the  process.  This 
difficulty  is  overcome  by  having  the  rubber  of  considerable  thickness ; 
the  author's  view  is  that  not  less  than  one-tenth  of  an  inch  radial 
thickness  should  be  employed,  and  he  would  prefer  to  have  one- 
eighth  of  an  inch,  or  more.  But  here  comes  in  the  other  difficulty — 
rubber  is  very  expensive ;  that  is  to  say,  good  rubber  is.  There 
are  two  principal  kinds  of  rubber  on  the  market,  known  respectively 
as  "  Para  "  and  West  African.  The  distinction  will  probably  very 
soon  disappear,  as  rubber  is  being  planted  in  Ceylon  and  in  various 
other  places,  and  there  are  very  good  accounts  of  the  quality  and 
quantity  of  the  new  rubber  produced.  At  the  present  time,  however, 
the  bulk  of  West  African  rubber  is  very  inferior,  and  is  worth  on 
the  market  only  about  one-fifth  that  of  Para.  Para  rubber  is 
grown  in  the  district  known  by  that  name  in  the  neighbourhood  of 
the  Kiver  Amazon,  and  it  is  not  only  very  much  better  in  quality 
when  produced,  but  the  natives  of  the  district  have  a  method  of 
preparing  it,  after  it  is  collected  from  the  trees,  which  kills  the 
parasite  that  is  present,  and  which,  if  not  destroyed,  disintegrates 
the  rubber  at  a  later  date.  The  rubber  insulation  of  all  cables 
employed  for  electric  light  and  power  distribution  should  consist  of 
at  least  thirty  per  cent,  of  Para.  West  African  rubber,  however,  is 
made  up  to  look  exactly  like  Para,  is  employed  for  insulating  cables, 
and  is  made  to  stand  all  the  tests  to  which  electrical  engineers  are 


2i8  ELECTRICITY   IN   MINING 

at  present  able  to  submit  it,  equally  as  well  as  Para,  but  the  useful 
life  of  the  West  African  rubber-covered  cable  is  only  a  fraction  of 
that  of  the  cable  with  the  percentage  of  Para  rubber  mentioned. 


Bitumen-covered  Cables 

The  high  cost  of  rubber  has  led  to  the  adoption  of  cheaper 
substances,  of  which  bitumen  is  the  one  that  has  been  most  largely 
employed  in  mines.  Bitumen  is  a  substance  known  commonly  as 
pitch,  which  is  found  naturally  in  the  pitch  lakes  of  Trinidad  and 
other  places.  Its  insulation  resistance  is  only  a  fraction  of  that  of 
rubber,  but  it  is  very  much  cheaper,  and  it  has  done  very  good 
service.  The  bitumen  is  first  purified.  In  its  natural  state  it 
contains  a  quantity  of  dirt  and  foreign  matter  which  would  prevent 
its  being  worked,  and  which  also  would  be  fatal  to  its  insulation. 
The  impurities,  dirt,  etc.,  are  removed  by  heating  and  straining,  and 
the  substance  is  then  used  for  insulating  cables  in  three  principal 
forms.  Messrs.  Callender  make  two  forms  of  insulators,  one  con- 
sisting of  paper,  or  spun  jute  fibre,  laid  over  the  insulator,  and  then 
thoroughly  impregnated  with  bitumen  and  oil  specially  prepared  for 
the  purpose,  a  lead  sheath  being  placed  over  the  insulator  under 
hydraulic  pressure.  They  also  insulate  cables  by  laying  what  they 
call  a  separator  of  yarn  or  paper  directly  over  the  conductor,  the 
wires  of  which  have  been  carefully  tinned,  and  outside  of  the 
separator  a  tube  of  vulcanized  bitumen  is  laid  on,  the  substance 
being  forced  down  on  to  the  cable,  through  a  dye,  as  the  cable  passes 
through  the  machine.  There  is  also  another  method  employed  by 
the  St.  Helens  Cable  Co.,  who  insulate  cables  with  a  substance  they 
have  called  "  dialite,"  consisting  of  bitumen  prepared  with  certain 
other  substances,  formed  into  sheets,  cut  into  strips,  and  laid  on  the 
cables  in  the  same  manner  as  the  pure  rubber  strip  described  in 
connection  with  rubber  cables.  The  dialite  cables  are  exposed  to 
a  certain  temperature  for  a  certain  time  after  the  insulator  has  been 
laid  on  the  conductor,  the  process  being  called  vulcanizing,  and 
causing  a  welding  together  of  the  layers  of  the  dialite,  and  this 
forming  into  one  homogeneous  envelope,  as  with  the  rubber  cables. 

It  is  claimed  that  dialite  stands  a  higher  temperature  than 
ordinary  vulcanized  bitumen,  and  that  it  will  withstand  oxidation 
and  the  action  of  the  salts,  that  are  so  frequently  found  in  the  water 
in  pit  shafts,  better  than  bitumen. 

All  three  forms  of  cable  are  sometimes  armoured,  the  armouring 
being  sometimes  of  wire,  one  or  two  layers,  sometimes  of  steel  tapes, 
one  or  two  layers,  and  they  are  sometimes  simply  braided.  Messrs. 
Callender  also  provide  a  particular  form  of  armour,  known  as  the 


f 
DISTRIBUTION   OF   POWER   BY  ELECTRICITY    219 

"  locked  coil."  It  is  taken  from  the  locked  coil  wire  rope  that  has 
been  upon  the  market  for  some  years,  which  is  well  known  to  mining 
engineers,  in  which  the  outer  strands  are  formed  of  a  particular 
section  arranged  to  dovetail  into  each  other,  and  to  form  together  a 
complete  cylindrical  envelope. 

Paper  and  Yarn  covered  Cables 

One  form  of  paper  and  yarn  covered  cables  has  already  been 
described,  that  by  Messrs.  Callender.  Paper,  yarn,  cotton,  and 
similar  substances  have  all  a  very  high  insulation  resistance  when 
absolutely  void  of  moisture.  Hence  they  form  very  good  and  very 
cheap  insulating  substances,  providing  that  the  moisture  can  be  kept 
out  of  them,  and  a  number  of  forms  of  cable  have  been  worked  out 
on  these  lines.  The  conductor  is  covered  in  some  cases  with  strips 
of  paper  laid  on  transversely,  and  so  that  the  joints  cross,  sometimes 
with  yarn  laid  on  in  a  similar  manner.  The  yarn,  or  the  paper,  first 
has  all  its  moisture  thoroughly  extracted  from  its  pores,  and  it  is  then 
thoroughly  impregnated  with  hydrocarbon  oils  under  pressure,  the 
whole  being  drawn  into  a  lead  tube.  The  rationale  of  the  arrange- 
ment is,  as  long  as  the  insulating  substance  has  its  pores  filled  with 
the  insulating  oil,  the  insulation  will  be  good,  but  if  the  paper  or 
yarn  is  exposed  to  the  atmosphere,  oxidation  immediately  commences 
and,  in  addition,  moisture  penetrates,  the  insulation  being  very  quickly 
destroyed,  hence  the  use  of  the  lead  tube  for  keeping  the  moisture 
out.  The  lead  tube  is  generally  formed  directly  on  the  cable,  the 
cable  passing  through  the  die  arranged  for  the  purpose.  In  some 
cases,  however,  the  insulated  conductor  is  drawn  through  a  tube 
already  made,  and  the  tube  is  squeezed  down  on  to  the  insulating 
material  by  hydraulic  pressure.  Messrs.  Glover  also  have  adopted 
another  method  of  applying  the  insulator.  They  prepare  a  stiff  paper 
cut  into  strips  and  impregnated  with  an  insulating  substance  they 
have  called  "  diatrine,"  which  imparts  a  sticky  surface  to  the  paper. 
The  paper  is  laid  on  diagonally,  as  already  described,  the  diatrine 
on  the  surface  of  the  paper  causing  the  successive  layers  to  adhere 
together,  and  the  lead  tube  is  then  formed  on  the  insulated  cable. 

A  variation  of  the  lead  tube  protecting  the  insulating  material  is 
a  tube  of  bitumen,  which  has  been  employed  both  by  Messrs.  Glover 
and  Messrs.  Callender. 

The  great  merit  of  the  paper  or  yarn  covered  cable  is  its  greater 
cheapness.  It  also  has  a  comparatively  high  resistance  to  sparking. 
The  substances  employed,  however,  should  be,  with  paper  at  any  rate, 
good  tough  fibrous  manilla,  and  it  should  be  thoroughly  well  dried 
before  applying,  as  explained.  There  is  the  same  possibility  of 


220  ELECTRICITY   IN   MINING 

failure  with  paper  or  yarn  covered  cables  as  with  rubber  covered,  if 
either  the  paper  or  yarn  is  poor.  If  not  of  the  best  quality  it  easily 
rots,  or  is  partially  destroyed  in  the  process  of  manufacture,  and  if 
the  impregnating  oils  are  impure,  or  contain  any  substance  which 
will  act  deleteriously  upon  the  paper,  the  same  thing  results. 

In  using  paper  or  yarn  covered  cables,  the  ends  must  never  be 
allowed  to  be  open  to  the  atmosphere,  otherwise  moisture,  which  is 
always  present  in  the  atmosphere,  will  gradually  creep  into  the  cable, 
and  will  destroy  the  insulation.  The  author  has  been  informed  also, 
by  a  cable-maker  of  experience,  of  a  case  where  a  paper-covered  cable 
was  suspended  in  the  shaft,  and  where  the  lead  covering  burst  near 
the  bottom,  in  his  opinion  owing  to  the  pressure  of  the  oil,  etc., 
above.  The  reply  to  this,  of  course,  is  that  a  stronger  lead  covering 
was  necessary. 

Paper-covered  cables  are  sometimes  covered  with  armour,  one  or 
two  layers  of  wire  or  steel  tapes,  and  sometimes  merely  braided. 


Fireproof  Covering  for  Cables 

One  of  the  troubles  in  connection  with  cables  is,  if  a  fall  of  roof 
occurs,  the  cables  being  broken,  and  the  bare  ends  lying  together,  so 
that  a  spark  followed  by  an  arc  passes  between  them,  the  heat 
generated  by  the  arc  ignites  the  insulating  envelope  of  the  cable, 
and  this  may  lead  to  other  very  serious  results.  The  same  thing 
may  and  has  happened,  where  a  switch  has  been  allowed  to  form  an 
arc  when  opened,  or  when  arcs  have  been  set  up  between  parts  of  a 
switchboard,  between  which  considerable  difference  of  pressure  exists, 
owing  to  the  deposit  of  coal  or  other  dust.  In  all  of  these  cases  the 
heat,  generated  by  the  arc  being  at  an  enormous  temperature,  ignites 
any  inflammable  material  near  it.  To  meet  this,  Messrs.  Glover  and 
Messrs.  Callender  have  introduced  a  covering  for  their  cables  which 
they  claim  to  be  fireproof.  There  is,  of  course,  no  substance  that  is 
actually  fireproof,  but  there  are  substances  which  ignite  with  great 
difficulty,  and  which,  when  ignited,  only  smoulder,  and  therefore  do 
not  communicate  combustion  readily  to  adjoining  substances,  and 
this  is  what  those  firms  have  provided.  After  the  cable  is  com- 
pleted it  is  covered  with  a  braid  of  a  substance  which  has  a  very  high 
ignition  point,  which  is  almost  non-combustible,  and  afterwards  the 
finished  cable  is  drawn  through  a  bath  of  a  substance  which  also 
has  a  very  high  ignition  point.  The  result  of  this  is  claimed  to  very 
considerably  decrease  the  dangers  mentioned  above. 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY    221 


The  Formation  of  Stranded  Conductors 

Conductors  are  formed  into  cables  by  stranding,  for  convenience 
in  handling.  Above  No.  12  gauge  the  conductor  becomes  stiff  and 
unpliable,  unless  it  is  of  a  very  soft  kind  of  copper,  which  it  is  not 
always  wise  to  employ,  and  in  the  author's  view  No.  14  is  the 
largest  single  wire  that  should  be  employed.  After  that  six  wires 
are  stranded  round  a  seventh,  forming  the  cables  known  as  7/16, 
7/18,  and  so  on,  the  stranded  form  being  made  in  every  size  right  down 
to  small  wires.  When  the  size  of  the  wire  required  with  seven  con- 
ductors for  a  given  total  sectional  area  becomes  large,  twelve  wires 
are  laid  on  the  outside  of  the  seventh,  and  another  set  of  cables  known 
as  19/16  and  19/18,  and  so  on,  are  formed.  With  still  larger  cables 
eighteen  wires  are  laid  on  outside  of  the  nineteenth,  and  another 
series  known  as  37/16,  37/18,  and  so  on,  are  formed.  For  still  larger, 
again,  twenty-four  wires  are  laid  on  outside  of  the  eighteenth,  and  a 
sixty-one  series  is  formed,  and  for  the  largest  size  thirty  wires  are 
laid  over  the  twenty-fourth,  and  the  ninety-one  series  is  formed.  In 
practice  cables  are  made  from  3/25  to  3/18,  from  7/24  to  7/6  from 
19/24  to  19/7,  from  37/24  to  37/8,  from  61/24  to  61/10,  and  from 
91/18  to  91/11. 

Another  point  that  has  to  be  considered  in  connection  with  cables 
is  makers'  lengths.  It  will  be  understood  that,  while  with  small- 
sized  cables  almost  any  length  can  be  made,  the  only  difficulty 
being  the  question  of  coiling  it  on  a  drum  of  sufficient  size,  as  the 
size  of  the  cables  becomes  larger  the  weights  becomes  heavier,  and 
the  whole  thing  becomes  more  difficult  to  handle,  and  therefore  only 
certain  lengths  can  be  employed.  In  laying  out  cables,  therefore,  for 
mines,  it  is  wise  to  choose  those  in  which  makers'  lengths  will  fit  in 
with  lengths  of  a  shaft.  On  no  account  should  there  be  joints  in  a 
shaft  if  they  can  possibly  be  avoided. 


Concentric  Cables 

For  convenience  in  handling,  the  cables  of  a  two-wire  con- 
tinuous current,  and  of  a  three-wire  continuous  current  system  are 
sometimes  made  into  one  cable,  in  the  forms  known  as  concentric 
and  triple  concentric.  In  the  concentric  cable  one  conductor  is  made 
in  the  ordinary  way,  stranded  as  usual,  and  insulated  to  its  full 
thickness,  the  insulator  being  further  protected  by  braiding  or  other 
arrangement.  Outside  of  the  insulation  of  the  first  conductor,  the 
second  conductor  is  laid  usually  in  one  layer,  the  number  of  wires  in 
the  single  layer  being  the  same  as  the  number  of  wires  in  the  stranded 


222 


ELECTRICITY  IN   MINING 


Section  of  Armoured  Lead 

Sheathed  Conductor 
Fia.  92. — Showing  Sections  of  Messrs.  Mavor  &  Coulson's  Concentric  Cables 


Section  of  Unarmourcd  Lead 
Sheathed  Conductor 


with  Uninsulated  Outers. 


FIG.  93.— Diagrams  showing  Method  of  Jointing  Messrs.  Mavor  &  Coulson's 
Concentric  Cables. 


DISTRIBUTION   OF  POWER   BY   ELECTRICITY     223 


conductor  on  the  inside,  or  the  total  sectional  area  of  the  wires  forming 
the  outer  conductor  being  equal  to  the  total  sectional  area  of  the 
wires  forming  the  inner  conductor.  Concentric  cables  are  sometimes 
built  up  of  copper  formed  into  sections,  something  on  the  lines  of  the 
locked-coil  armour  mentioned,  or  in  the  form  of  sectors  of  cylinders, 
but  in  all  cases  the  outer  conductor  completely  surrounds  the  insu- 
lating envelope  of  the  inner  conductor,  and  the  sectional  area  of  the 
two  is  exactly  equal. 

Triple  concentric  cables  are  for  the  three-wire  system,  and  in 
them  the  outer  conductor  of  a  concentric  cable  is  insulated,  and  a 
third  conductor  is  laid  on  outside.  "With  triple  concentric  cables 
the  outer  conductor  is 
usually  made  the  neu- 
tral. It  is  very  much 
smaller  than  either  of 
the  others.  It  is  usual 
to  insulate  the  outer 
conductor  outside  of  all, 
and  the  completed  cable 
may  be  armoured  with 
wire  or  strip  steel,  or 
may  simply  be  braided, 
or,  again,  may  be  drawn 
into  a  lead  tube. 

There  is  a  modifica- 
tion of  the  concentric 
cable  which  has  been 
largely  used  in  the 
Scottish  collieries,  intro- 
duced by  Messrs.  Mavor 

Plan  of  Joint  Box  and  Conductor 


FIG.  94. — Showing  Method  of  connecting  a  Branch 
to  Messrs.  Mavor  &  Coulson's  Concentric  Cables. 


&  Coulson,  in  which 
the  outer  conductor  is 
uninsulated.  The  outer 
conductor  consists 

partly  of  galvanized  iron  wires,  partly  of  a  lead  tube  enclosing  the 
insulating  envelope  of  the  inner  conductor,  and  partly  of  a  copper 
conductor  introduced  to  increase  the  conductivity.  The  outer  con- 
ductor in  this  case  forms  the  return  or  negative  conductor.  It  is 
only  employed  with  continuous  currents,  and  it  is  claimed  that  it 
affords  a  very  efficient  protection  against  shock.  All  junction  boxes, 
switchboxes,  etc.,  are  arranged  so  that  the  outer  conductor  makes 
good  electrical  connection  with  the  containing-box.  The  author's 
objection  to  this  is,  that  if  a  cable  is  parted,  say  by  a  fall  of  roof,  it  is 
difficult  to  make  an  efficient  connection  to  the  outer  conductor.  Figs.  92, 
93,  and  94  show  these  cables,  and  the  method  of  jointing  them. 


224  ELECTRICITY   IN   MINING 


Three-core  Cables 

Three-core  cables  are  made  for  use  with  three-phase  currents. 
It  was  explained  in  Chapter  I.  that  induction  takes  place  between 
cables  in  which  currents  are  passing  when  the  strengths  of  the 
currents  are  changing,  and  in  order  to  neutralize  the  inductive  effects 
as  much  as  possible,  the  cables  for  a  three-phase  system  must  be 
brought  as  close  together  as  possible,  and  if  they  can  be  twisted 
round  each  other,  the  neutralizing  effect  will  be  very  much  increased. 
Further,  it  is  absolutely  necessary  that  no  single  cable  carrying  an 
alternating  current  shall  be  laid  by  itself  with  its  own  armour,  the 
reason  being  that  the  induction  taking  place  in  the  armour  of  the 
cables  will  be  so  great  as  to  very  seriously  affect  the  efficiency  of 
the  system,  a  large  amount  of  the  power  delivered  to  the  cable  being 
swallowed  up  by  the  induction  in  the  iron  armour.  Hence  a 
convenient  arrangement  for  three-phase  work  is,  each  cable  is  insu- 
lated in  the  manner  intended,  the  insulation  is  braided,  and  the 
three  cables  are  laid  up  together,  usually  round  a  hemp  core, 
the  spaces  formed  by  the  stranding  of  the  three  cables  are  filled  in 
with  yarn  or  other  substance,  and  the  whole  is  insulated  outside  of 
all,  the  completed  cable  then  being  either  armoured,  drawn  into  lead 
pipe,  or  simply  braided,  as  may  be  desired.  The  armour  in  this  case, 
or  the  lead  pipe,  being  common  to  all  the  cables,  and  equidistant 
from  all  of  them,  any  inductive  effects  will  be  practically  neutralized. 

For  two-phase  service,  four  cables  may  be  made  into  one,  or,  as  is 
more  frequently  done,  twin  cables  are  employed,  each  cable  of  each 
twin  being  separately  insulated,  and  the  twins  being  insulated  over 
all  as  well.  The  author  does  not  like  twin  cables ;  his  experience  has 
been  that  they  are  very  liable  to  breakdowns. 

Sizes  of  Cables  for  Lighting  and  Power 

The  sizes  of  cables  for  lighting  and  power  services  are  controlled 
by  three  factors — the  heating  factor,  that  of  waste  of  power,  and  drop 
of  pressure.  As  explained,  when  a  current  is  passing  through  a  pair 
or  through  three  cables,  as  the  case  may  be,  to  work  lamps  or  motors 
beyond  it,  the  pressure  falls  continuously  between  the  two  cables  as 
the  generator  becomes  more  and  more  distant,  in  exact  proportion  to 
the  formula  E  =  CK,  where  E  is  the  fall  of  pressure,  C  is  the  current 
passing,  and  K  is  the  resistance  up  to  the  point  where  the  measure- 
ment is  made.  The  loss  or  waste  of  power  in  the  cables  is  measured 
by  the  formula  W  =  EC,  where  E  is  the  drop  in  pressure  between 
the  two  pairs  of  ends  of  the  cables,  and  C  is  the  current  passing. 
Evidently  the  proportion  of  fall  in  pressure  will  be  the  proportion  of 


DISTRIBUTION   OF   POWER   BY  ELECTRICITY    22$ 

loss  in  the  cables.  Thus,  if  the  pressure  at  the  terminals  of  the 
generator  is  500  volts,  and  the  pressure  at  any  point  —  say  the  pit 
bottom,  or  a  distributing  point  further  on  —  through  which  all  the 
current  passes,  is  450  volts,  the  loss  in  transmission  through  the 
cables  is  ten  per  cent.  Further,  this  rule  applies  as  the  distribution 
goes  on,  each  branch  wastes,  or  charges  upon  the  power  delivered  to 
it,  in  proportion  to  the  ratio  between  the  fall  of  pressure  and  the 
pressure  at  the  commencement  of  the  branch.  The  cables,  however, 
are  the  one  portion  of  the  apparatus  in  which  the  engineer  is  master. 
Within  certain  wide  limits  he  can  make  the  loss  in  the  cables  as 
little  or  as  great  as  he  chooses.  A  loss  of  ten  per  cent,  between  the 
generator  and  the  bulk  of  the  work  is  commonly  accepted  as  a 
standard  ratio  ;  but  this  is  by  no  means  a  hard-and-fast  rule,  and  the 
proportion  of  power  that  may  be  wasted  in  the  cables  depends  entirely 
upon  the  conditions  of  the  service.  Over  twenty  years  ago  Lord 
Kelvin  stated  the  law  that  for  greatest  economy  the  loss  in  any 
cables  must  equal  the  cost  of  the  horse-power  expended  in  them. 
This  law  has  since  been  modified,  and  the  modern  reading  may  be 
taken  to  be  :  the  loss  in  cables  in  horse-power  may  be,  for  economy, 
such  an  amount  that  the  annual  cost  of  the  horse-power  equals  the 
annual  cost  of  the  cables  it  displaces.  The  first  cost  of  cables  is 
made  up  of  the  cost  of  the  copper,  and  the  insulation,  and  manu- 
facture generally,  plus  the  cost  of  fixing.  The  interest  on  the  first 
three  items,  added  to  the  cost  of  maintenance,  marks  the  limiting 
value  to  which  that  of  the  horse-power  may  go  before  its  waste 
becomes  uneconomical.  It  will  be  seen  from  this  that  where  power 
is  generated  very  cheaply  indeed,  as  in  a  few  cases  of  water  power, 
or  by  sources  of  natural  gas,  by  the  use  of  otherwise  waste  products, 
etc.,  it  may  be  economical  to  waste  a  very  large  proportion  of  the 
total  power  generated,  because  this  may  enable  very  much  smaller 
cables  to  be  employed,  and  the  interest  and  upkeep  of  them,  etc., 
to  be  materially  decreased. 

With  continuous-current  systems  the  rule  for  the  calculation  of 
the  size  of  the  cables  is  as  follows.  Determine  the  pressure  in  volts 
that  may  be  wasted  in  the  particular  cables  whose  size  is  to  be 
calculated,  and  then  apply  the  formula  — 

176Q 


where  K  is  the  resistance  per  mile  of  the  cables,  which  may  be  taken 
from  any  manufacturer's  catalogue,  E  is  the  pressure  in  volts 
expended  in  that  particular  pair  of  cables,  C  is  the  largest  current 
the  cables  are  to  transmit,  and  L  is  the  length  of  the  two  cables. 

For  three-phase  cables  the  formula  has  to  be  modified.     In  the 
three-phase  system  each  pair  of  cables  with  the  conductor  on  the 

Q 


226  ELECTRICITY  IN   MINING 

armature  of  the  generator  between  them  may  be  considered,  for 
the  purposes  of  distribution,  as  a  separate  machine,  and  the  formula 
then  works  out  for  each  individual  cable — 

1760  x  1-71 

~2&x~ir' 

L  being  the  resistance  of  one  cable. 

For  the  two-phase  system  each  pair  of  cables  may  be  considered 
to  be  taking  half  the  current,  and  the  calculation  may  be  made  for 
each  half,  as  for  continuous  currents. 

A  caution  should  be  given  here  in  connection  with  two  and  three 
phase  currents,  viz.  that  the  power  factor  which  has  been  so  often 
referred  to,  must  be  taken  into  account.  Current  has  to  be  generated, 
and  has  to  be  carried  by  the  cables,  in  addition  to  that  actually 
employed  in  working  motors,  etc.,  "idle"  current,  as  it  is  termed, 
owing  to  the  lag  which  was  described  in  Chapter  I.  This  means  that 
the  cables  have  to  be  made  larger  than  they  otherwise  would  do  to 
allow  for  the  idle  current,  and  in  applying  the  formula  for  the  size  of 
the  cable,  the  power  factor  taken  at  0-8  as  a  standard,  unless  there  are 
special  reasons  for  taking  it  at  any  other  figure,  must  be  applied  to 
the  equation. 


Heating  of  Cables 

It  was  explained  in  Chapter  I.  that  the  heat  unit  is  connected 
with  the  electrical  system  of  units  by  the  fact  that  17*58  watts  =  1 
B.Th.  Unit ;  that  is  to  say,  if  an  electric  current  is  delivering  energy 
at  the  rate  of  17'58  watts,  no  matter  at  what  pressure,  to  any  con- 
ductor, heat  is  liberated  in  the  conductor  at  the  rate  of  1  unit  per 
minute.  It  was  also  explained  that  the  heat  liberated  is  measured 
by  the  three  formulae — 

H  =  ECt, 
H  =  C2K£, 
and — 

TT   -m 

=  TT 

The  last  two  are  the  important  formulae  for  this  purpose,  and  from 
them  it  will  be  seen  that  the  heat  liberated  in  any  given  cable  of  a 
given  resistance,  in  any  given  time  ty  varies  directly  as  the  square  of 
the  current  strength  in  amperes,  and  also  directly  as  the  square  of 
the  pressure  in  volts.  The  actual  quantity  of  heat  liberated  in  any 
time  in  any  cable  may  be  found  by  adding  to  the  above  equations 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     227 

the  weight  of  the  cable  in  pounds,  and  the  specific  heat  of  copper, 
and  the  increase  of  resistance  of  the  copper  due  to  its  increased 
temperature.  It  was  explained  in  Chapter  I.  that  the  resistance  of 
the  metals  increases  with  increased  temperature  in  a  definite  ratio, 
that  of  copper  being  '004  per  degree  C.  When  a  current  is  passing 
through  any  cable  under  ordinary  working  conditions,  heat  is 
liberated  in  the  conductor,  and  a  certain  portion  of  it  is  dissipated 
through  the  insulating  envelope  to  the  surrounding  atmosphere. 
Under  certain  conditions,  where  the  current  density  in  the  conductor 
is  very  low,  and  where,  as  in  most  mines,  there  is  a  powerful  current 
of  air  passing  along  the  surface  of  the  cable,  there  would  be  no 
appreciable  rise  of  temperature  in  the  conductor,  and  the  heat 
liberated  would  be  measured  by  the  formula — 

CPRt 
17-58 

But  where  the  current  density  is  high,  and  where  the  heat  has  no 
opportunity  of  escaping,  as  where  conductors  are  enclosed,  or  are 
coiled  on  each  other,  as  in  dynamo  machines,  there  is  a  certain 
definite  increase  of  temperature,  and  also  a  certain  definite  increase 
of  resistance.  The  importance  of  this  question  lies  in  the  fact  that 
conductors  for  any  purpose  must  not  carry  more  than  a  certain 
current  density.  The  insurance  companies,  the  Institute  of  Electrical 
Engineers,  and  others,  have  settled  a  standard  density  of  1000 
amperes  per  square  inch  of  sectional  area;  but  under  certain  con- 
ditions this  density  may  be  very  greatly  exceeded,  while  under  other 
conditions  it  is  wiser  not  to  come  up  to  it.  With  overhead  conductors 
which  are  exposed  to  the  atmosphere,  and  especially  where  there  are 
usually  fairly  strong  prevailing  winds,  the  current  density  may  be 
many  times  higher  than  the  standard.  The  question  of  economy 
usually  dictates  a  low  current  density,  but  there  are  cases  where  it 
may  be  economical  to  adopt  a  high  current  density,  and  in  those 
cases  it  is  perfectly  safe  to  do  so,  providing  that  the  heat  which  is 
delivered  to  the  conductor  can  escape  from  its  surface  at  such  a  rate 
that  the  mass  of  the  conductor  maintains  its  position  inside  its 
insulating  envelope. 

The  heating  effect   becomes  important  when  either  leakage  or 
short  circuits  take  place.     The  formula — 

m 
"S- 

shows  that,  with  a  given  conductor,  the  heat  liberated  will  depend 
directly  upon  the  square  of  the  pressure.  When  a  cable  is  merely 
carrying  a  current  to  an  apparatus,  or  to  other  cables  beyond  it,  only 


228  ELECTRICITY   IN    MINING 

a  very  small  pressure  is  present  between  its  ends  ;  but  when  connec- 
tion is  made,  say,  between  the  two  cables  of  a  continuous  current 
system  at  the  pit  bottom,  then  the  whole  of  the  pressure  of  the 
system  becomes  available  for  delivering  heat  to  the  two  conductors  in 
the  shaft,  and,  as  will  be  seen,  the  result  will  be  an  enormous  increase 
in  the  rate  at  which  heat  is  liberated.  A  simple  calculation  shows  the 
difference  in  the  possible  heat  liberated  in  a  pair  of  conductors  when 
50  volts  of  a  total  pressure  of  500  are  expended,  and  when  the  whole 
pressure  is  expended,  in  shaft  cables.  If  the  cables  are  short  circuited, 
the  full  500  volts  are  available  for  delivering  heat  in  the  conductors. 
The  increase  of  heat,  if  there  were  no  change  in  the  resistance  of  the 
conductor  or  in  the  pressure,  would  be  as  502  to  5002,  as  25  to  2500 
approximately,  or  the  heat  liberated  would  be  100  times  as  great. 
The  increase  of  resistance  would  decrease  this  proportion,  and  also 
the  enormous  current  that  would  be  delivered  by  the  generator  would 
tend  to  lower  the  pressure  at  its  terminals,  both  by  lowering  the 
speed  of  the  engine,  and  by  lowering  the  pressure  electrically.  But 
it  is  easy  to  see,  and  a  simple  calculation  will  show,  that  in  a  pair 
of  cables  consisting,  say,  of  19/16  wires,  feeding  a  500-volt  service, 
a  short  circuit  at  the  bottom  of  a  pit  400  yards  deep  might  raise  the 
temperature  of  the  conductor  to  melting-point,  if  allowed  to  operate 
for  a  comparatively  short  time. 


Wires  and  Cables  for  Connecting  to  Lamps,  etc. 

Small  wires  and  small  cables  for  connecting  to  incandescent,  to 
arc  lamps,  and  to  small  motors,  switches,  etc.,  had  better  always  be 
insulated  with  rubber  as  described  on  page  216,  the  rubber  being 
covered  outside  of  all  with  a  substantial  coating  of  jute.  If  the  cables 
are  to  be  fixed  in  places  where  they  will  be  liable  to  damage,  it 
will  be  wise  to  have  the  rubber  as  thick  as  the  means  will  allow. 
For  the  inside  of  engine  houses,  offices,  and  similar  places,  the  wires 
and  small  cables  may  be  fixed  in  boxing,  consisting  of  wood,  in  which 
grooves  are  run  for  the  wires  to  lie,  a  cover  being  fixed  over  them 
when  in  place,  and  the  grooves  being  just  large  enough  to  allow  the 
wires  to  be  lightly  tapped  in.  The  casing  before  being  used  should 
be  thoroughly  dried,  and  should  have  either  several  coats  of  shellac 
varnish,  each  coating  being  well  dried  before  the  next  is  applied,  or 
be  protected  from  moisture  in  some  similar  manner.  Wires  and 
small  cables  may  also  be  protected  by  a  conduit  which  has  been 
introduced  from  America,  called  the  circular  loom  conduit.  It  is 
made  entirely  of  insulating  material  with  a  braiding  on  the  outside, 
woven  in  a  specially  strong  manner,  the  whole  thing  being  steeped 
in  insulating  substances,  of  which  finely  divided  mica  forms  a  part, 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     229 

and  the  conduit  having  considerable  strength.  This  conduit  will, 
in  the  author's  opinion,  be  found  of  great  service  for  a  great  many 
places  about  the  mine,  as  well  as  about  pit  tops,  engine  houses, 
fitting  shops,  etc.  Casing  also,  made  from  wood  that  has  been 
subject  to  one  of  the  recently  introduced  fireproofing  processes,  such 
as  "  haskinizing,"  should  answer  well. 


Cables   for  Coal=cutting  Machines  and    Moving 

Motors 

The  cables  employed  to  follow  coal-cutting  machines,  or  pumps 
on  a  dip  road,  present  considerable  difficulty.  In  the  case  of  a  coal- 
cutting  machine  cutting  across  a  coal  face  of,  say,  1000  yards,  it  is 
necessary  to  have  a  certain  length  of  cable  trailing,  and  it  is  best 
that  the  cables,  whether  the  motor  is  continuous  current  or  three 
phase,  should  be  formed  into  one.  The  conductors  of  which  the  cables 
are  composed  should  be  made  very  flexible  ;  that  is  to  say,  they  should 
be  made  of  a  comparatively  large  number  of  smaller  wires  than 
would  be  usual  if  the  cable  were  simply  delivering  current  in  the 
ordinary  way.  The  larger  the  number  of  wires  of  which  each  con- 
ductor is  formed,  the  more  flexible  will  it  be,  but,  on  the  other  hand, 
the  more  easily  will  it  be  parted  if  wet  penetrates  to  it.  The  two 
or  three  cables  should  be  laid  up  together,  and  they  may  be  held 
together  by  a  light  steel  wire  armour,  or,  as  the  author  prefers,  a 
wrapping  and  a  braid  of  jute,  or  the  plaited  leather  covering  that  has 
been  introduced  by  Messrs.  Glover.  The  gate  road  connecting  boxes 
for  coal-cutting  machines  should  be  arranged  at  as  frequent  intervals 
as  possible,  in  order  that  the  trailing  cable  may  be  as  short  as 
possible ;  but,  on  the  other  hand,  this  entails  carrying  the  supply  cable 
either  up  several  gate  roads,  or  across  the  face.  No  rule  can  be 
given  applicable  to  all  cases  of  this  kind,  the  electrician  at  the 
colliery  must  use  his  own  judgment,  following  the  rule  given  on 
page  235,  to  keep  the  cables  out  of  the  way  of  everything  as  much 
as  possible. 

Fixing  Cables  in   Mines 

The  most  important  part  of  the  mine  with  respect  to  the  fixing 
of  cables  is  the  shaft.  Even  with  small  insulated  wires  such  as  are 
employed  for  signals,  the  problem  of  fixing  them  in  the  shaft  in  such 
a  manner  that  they  shall  not  be  damaged  by  falling  mineral,  and 
other  substances  is  by  no  means  an  easy  one.  As  the  conductors 
become  heavier,  as  they  do  with  electric  light  and  power  services,  the 


23° 


ELECTRICITY  IN    MINING 


problem  becomes  more  and  more  difficult,  because  the  weight  of  the 
cable  itself  and  the  effect  upon  the  elongation  of  the  conductor,  and 
the  possible  opening  of  the  insulating  material  comes  into  play. 
There  is  considerable  difference  of  opinion  as  to  whether  cables,  both 
in  the  shaft  and  on  the  roads,  should  be  armoured  or  not.  In  the 
author's  opinion,  which  he  has  published  on  every  possible  occasion, 
and  which  he  has  seen  no  reason  to  change,  armouring  is  wrong 
except  in  the  special  case  of  three-core  cables  for  three-phase 
currents,  and  then  he  would  only  allow  a  light  armour  for  the 
purpose  of  holding  the  three  cables  together  conveniently,  and  sup- 
porting them.  With  cables  intended  for  continuous  currents  he 
prefers  that  there  should  be  no  armour,  and  he  objects  to  the  use  of 


FIG.  95. — Showing  Messrs.  Callender's  Single  Cable  Cleat  for  bolting  to  a 
Brickwork  Shaft.  It  will  be  noticed  that  the  Cleat  is  kept  clear  of  the 
Brickwork. 

any  conductor  outside  of  the  insulator,  and  for  the  reason  that  it  is 
very  difficult,  in  manufacturing,  to  avoid  straining  the  insulating 
material  while  laying  the  outer  conductor  on.  With  the  advance  of 
manufacture  this  difficulty  has  been  to  a  large  extent  overcome,  and 
there  is  not  now  the  danger  there  was,  of  damaging  the  insulation. 
But  there  is  still  the  same  danger  of  damaging  the  insulation,  both 
in  the  process  of  laying  the  cable,  and  after  the  cable  is  laid,  par- 
ticularly in  mines,  where  cables  are  necessarily  subject  to  rough 
usage,  the  bending,  kinking,  squeezing,  etc.,  tending  to  drive  the 
armour  or  outer  conductor  through  the  insulating  material.  It  is  as 
well  to  remember  that  if,  in  the  case  of  an  armoured  cable,  the  armour 
is  driven  through  the  insulating  material  and  makes  connection  with 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     231 

the  copper  conductor,  the  armour  itself  becomes,  to  all  intents  and 
purposes,  the  conductor.  It  is  alive  and  will  give  shocks  if  it 
happens  to  be  the  positive  conductor.  It  will  also  tend  to  deliver 
currents  to  other  conductors  with  which  it  is  in  contact,  and  it  will 
also  tend  to  set  up  sparking,  not  only  between  itself  and  the  other 
conductors,  but  between  other  conductors  with  which  it  is  in  contact 
at  certain  points,  and  conductors  with  which  they  are  in  contact,  and 
from  which  they  may  be  temporarily  disconnected.  It  is  hardly 
necessary  to  enlarge  upon  the  danger  of  this.  For  fixing  cables  in  a 
shaft  there  are  broadly  four  methods. 

1.  The  cable  may  be  suspended  from  top  to  bottom  of  the  shaft 
without  any  intermediate  support.  This  method  may  be  adopted  in 
shallow  mines,  and  with  light  conductors,  and  providing  that  they 
are  properly  insulated  at  both  top  and  bottom,  and  that  the  bottom 
connection  is  carefully  protected  from  the  lodgment  of  water  and 
coal-dust.  For  many  cases  this  method  would  be  very  suitable.  The 


TUBBING 


FIG.  96. — Showing  Messrs.  Callender's  Cable  Cleat,  for  two  Cables,  for 
bolting  to  the  Brickwork  of  a  Shaft. 

great  danger  of  this  method  is  the  possibility  of  the  insulating 
envelope  of  the  cable  being  damaged  by  falling  coal,  and  the  lodg- 
ment of  water  and  coal-dust  where  the  cable  is  fixed  at  the  bottom, 
and  where  it  is  probably  bent,  and  where  the  insulating  envelope 
may  be  strained,  the  result  being  that  the  water  may  penetrate  the 
insulating  envelope. 

2.  A  modification  of  1  is,  the  cables  are  supported  by  insulators 
at  the  top  of  the  pit,  they  are  stretched  from  top  to  bottom  in  the 
same  manner  as  in  1,  and  they  are  further  supported  at  equal 
intervals  between  the  top  and  the  bottom  by  various  devices,  such  as 
short  pieces  of  wood  casing  secured  to  the  byatt,  with  a  cover 
arranged  to  squeeze  the  cable  into  a  groove  provided  for  it,  or  glazed 
earthenware  insulators  may  be  employed,  the  insulators  being  made 
in  two  halves  so  as  to  clasp  the  cable,  the  insulators  with  the  cables 
being  supported  by  brackets  secured  to  the  side,  or  to  the  byatt. 


232 


ELECTRICITY   IN   MINING 


This  method  is  also  open  to  •  the  objection  that  the  cable  may  be 
damaged  by  falling  coal,  etc.,  and  each  support  provides  a  lodgment 
for  water  and  coal-dust,  and  there  is  the  danger  of  trouble  arising  at 
those  points.  If,  however,  attention  is  given  to  these  matters,  and  the 
coal-dust  is  cleaned  off  at  fairly  frequent  intervals,  the  danger  is 
minimized,  and  the  results  should  be,  and  have  been,  satisfactory. 
Figs.  95,  96,  and  97  show  methods  of  supporting  cables  in  the  shaft 
by  wood  cleats,  as  arranged  by  Messrs.  Callender. 

3.  In  the  third  method,  which  in  the  author's  opinion  is  the  best, 

provided  that  it  is 
properly  carried  out, 
but  which  has  the 
usual  drawback  that 
it  is  more  expensive, 
wood  boxing  is  fixed 
against  either  the  side 
of  the  shaft  or  at  the 
back  of  the  byatts, 
the  boxing  being  made 
from  substantial 
planking  that  has  been 
subject  to  one  of  the 
preservative  processes 
such  as  haskinizing, 
the  grooves  for  the 
cables  being  made  so 
that  the  cables  them- 
selves have  to  be 
tapped  gently  into  the 
grooves,  and  are  then 
held  by  the  boxing 
the  whole  way  down 
the  shaft.  The  cover 
of  the  casing  in  this 
case  need  only  be 
sufficiently  strong  to 
prevent  falling  mineral 
from  knocking  it  away 

and  exposing  the  cables.  One  great  objection  to  the  use  of  wood 
boxing  for  holding  the  cables  in  the  shaft  is,  unless  the  wood  is 
treated  in  some  way,  it,  being  porous,  absorbs  water  like  a  sponge, 
and  as  it  clasps  the  cable  the  whole  way  down,  it  is  in  the  very  best 
position  to  deliver  the  water  with  any  salts  it  may  contain  to  the 
insulating  envelope,  and  as  the  water  is  always  there  and  always 
acting,  the  results  are  sometimes  serious.  Haskinising  is  claimed  to 


FIG.  97.— Messrs.  Calender's  Single  Cable  Cleat,  for 
Mine  Shafts,  supported  by  Chains  from  the  Brick- 
work. 


PLATE  14 A.— Ferranti  1500  Ohm, 
10,000  Volt,  Three  Phase,  Oil 
Enclosed,  Electrically  Operated 
Switch,  with  Switch  closed,  but 
with  Case  open. 


PLATE  14B.— Ferranti  1500  Ohm, 
10,000  Volt,  Oil  Enclosed,  Elec- 
trically Operated,  Three  Phase 
Switch,  with  Switch  open,  and 
with  Oil  Tanks  of  two  of  the 
Switches  lowered. 


PLATE    14c.— Gas  Proof,  Oil    En- 
closed, Three  Phase  Switch. 


PLATE  14D. — Ferranti  Gas  Proof, 
Three  Phase  Mining  Switch.  The 
Cover  is  up,  to  show  the  Contacts 
inside. 

[To  face  p.  232. 


DISTRIBUTION    OF   POWER   BY   ELECTRICITY     233 

prevent  all  this.  The  wood  before  being  subjected  to  the  process  is 
thoroughly  dried,  all  the  sap  removed,  and  all  moisture,  and  the  pores 
filled  with  a  substance  which  it  is  claimed  increases  the  insulating 
value  of  the  wood,  renders  it  non-combustible,  and  impervious  to 
moisture.  A  cheaper  form  of  this  method  is,  boxing  is  made  from 
substantial  planking  as  before,  it  is  thoroughly  dried — this  is  the 
most  important  point  in  the  whole  matter — and  it  is  treated  before 
being  placed  in  the  shaft  to  two  or  more  coatings  of  Stockholm  tar, 
the  treatment  with  tar  being  repeated  after  the  cables  are  fixed 
and  at  intervals  after  they  have  been  put  into  service.  This  method, 
the  author  understands,  has  also  met  with  considerable  success. 


FIG.  98. — Showing  Messrs.  Glover's  Method  of  jointing  a  Cable  in  a 
Shaft.  A  Recess  is  cut  in  the  Side  of  the  Shaft  and  the  Joint 
made  there. 

4.  The  fourth  method  is,  iron  pipes  are  fixed  in  the  shaft, 
secured  by  brackets,  or  in  any  convenient  way,  to  the  sides  or  to  the 
byatts,  and  the  cables  are  run  inside  them.  With  this  method  it  is 
also  sometimes  arranged  to  lay  the  cables  in  sections  in  the  shaft,  of 
any  convenient  length,  providing  junction  boxes  consisting  of  cast 
iron,  into  which  the  pipes  screw,  and  which  have  covers  arranged  to 
fix  on  their  fronts,  so  as  to  exclude  moisture.  If  iron  pipes  are  used, 
probably  this  is  as  good  a  method  as  can  be  adopted,  as  the  junction 


234  ELECTRICITY   IN   MINING 

boxes  provide  means  of  testing  the  different  sections  in  case  of  faults 
in  the  cables ;  but  in  the  author's  view  it  is  wrong  to  conceal  your 
cables  inside  iron  pipes.  Cables  should  always  be  either  visible, 
or  easily  got  at  for  inspection.  Further,  in  his  opinion  it  would 
probably  be  exceedingly  difficult  to  ensure  that  the  junction  boxes 
shall  always  be  watertight.  In  his  experience,  making  a  joint  in  a 
cable  or  wire  in  a  shaft  so  that  moisture  is  excluded  is  absolutely 
impossible.  He  has  never  yet  seen  any  shaft  in  which  cables  can  be 
fixed  in  which  moisture  was  not  present ;  while  in  the  great  majority 
of  mine  shafts  water  is  fairly  abundant,  to  such  an  extent  that  it  is 
very  difficult  indeed  to  keep  it  from  the  hands,  and  from  any  con- 
ductor that  one  may  be  handling  in  the  shaft.  As  explained  above, 
with  alternating  currents,  if  iron  pipes  are  employed,  all  the  cables 
must  be  enclosed  in  the  one  pipe. 

With  three-phase  cables  the  author  prefers  the  three-core  cable 
with  a  light  armour,  sufficient  to  take  the  weight  of  the  cable  to  a 
certain  extent,  and  he  would  prefer  its  being  fixed  in  wood  boxing, 
as  explained.  Fig.  98  shows  the  method  adopted  by  Messrs.  Glover 
for  jointing  a  cable  in  a  shaft. 


Fixing  Wires  on  Engine  Roads,  etc. 

For  engine  roads  again  there  are  several  methods.  The  use  of 
iron  pipes  arranged  in  a  somewhat  similar  manner  to  those  explained 
for  the  shaft  have  been  used  and  advocated,  the  pipes  being  placed 
on  the  floor  of  the  mine.  The  author's  objection  to  this  is  the  same 
as  in  the  case  of  the  shaft  cables :  you  cannot  see  what  is  happening 
to  your  cables,  and,  in  addition,  as  mining  engineers  know,  your  pipe, 
if  laid  on  the  ground,  is  apt  to  disappear,  and  is  subject  to  the 
working  of  the  mine,  causing  breaks  at  junctions,  etc.,  and  water 
may  get  into  pipes,  will  run  to  the  lowest  point,  and  will  there 
surely  cause  trouble.  Another  method  is  to  lay  the  cables  in  wood 
troughing,  and  to  fill  the  troughing  in  with  melted  pitch.  Where  it 
can  be  adopted,  this  plan  is  a  very  good  one,  and  it  does  not  matter 
what  the  wood  troughing  is  made  of,  providing  there  is  plenty  of 
space  for  pitching,  nor  does  it  matter  if  the  wood  troughing  disappears, 
as  the  pitch  tube  which  is  formed  is  a  very  good  protection  indeed 
for  the  insulating  envelope  of  the  cables.  Pitch  has  the  peculiar 
property — possessed  also  by  ice — of  sealing  up  any  cracks  or  openings 
that  are  formed,  say,  by  the  working  of  the  mine,  so  that  it  should 
be  difficult  for  moisture  to  penetrate  ..to  the  cable  in  any  appreciable 
quantities.  The  difficulty  of  applying  this  method  is,  that  in  many 
parts  of  coal  mines  it  is  not  possible  to  apply  sufficient  heat  to  melt 
the  pitch.  Another  method  is  to  support  the  cables  on  glazed 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     235 

earthenware  insulators,  which  are  fixed  to  the  props,  or  in  any  con- 
venient manner,  the  cables  being  tied  to  the  insulators  with  yarn  or 
similar  material.  This,  in  the  author's  opinion,  is  also  a  very  good 
method  of  fixing  cables  on  main  roads,  and  places  where  they  are  not 
very  liable  to  accident.  The  insulators  need  not  be  at  all  elaborate. 
Those  described  as  being  employed  for  terminating  iron  engine-road 
signal  wires,  which  have  a  groove  around  them,  will  answer  very 
well,  the  insulators  being  merely  fixed  to  the  prop  by  a  bolt,  and,  if 
necessary,  a  washer,  the  cable  lying  in  the  groove  on  the  insulator, 
and  being  secured  there  by  yarn.  In  another  method,  which  has  a 
great  deal  to  recommend  it,  and  which  is  applicable  either  to  main 
roads  or  to  gate  roads,  a  leather  thong  is  attached  to  the  cable  by  a 
loop,  and  its  other  end  is  nailed  or  bolted  to  the  prop,  or  any  con- 
venient spot.  The  great  advantage  claimed  for  this  method  is,  in 
case  of  a  fall  of  roof  the  leather  thong  is  broken,  and  the  cable  falls 
to  the  ground,  and  is  only  exposed  to  any  crushing  action  that  may 
take  place  after  the  roof  has  fallen,  not  to  any  cutting  action  such  as 
that  by  the  sharp  edge  of  a  piece  of  rock  cutting  through  the  insulating 
material  during  the  fall.  There  are  modifications  of  this  method  that 
will  be  obvious,  such  as  supporting  the  cables  by  bands  of  yarn  from 
props  or  hooks  supported  by  props  or  beams.  In  fact,  in  gate  roads, 
and  in  the  neighbourhoods  where  the  mine  is  working  more  or  less, 
almost  any  method  may  be  employed  that  will  keep  the  cables  apart, 
out  of  the  way  of  the  mineral  waggons,  and  that  will  protect  them,  as 
far  as  possible,  from  falls. 

Methods  of  Distribution 

One  of  the  important  points  for  consideration  in  laying  out  a 
power  plant  for  distribution  by  electricity  is,  the  pressure  at  which 
the  service  shall  be  worked.  It  will  be  understood,  from  what  has 
been  stated,  that  since  the  power  is  measured  by  the  product  of 
the  current  and  the  pressure,  if  the  pressure  can  be  increased,  the 
current  can  be  decreased,  and  by  decreasing  the  current,  the  size  of 
the  cables  for  the  transmission  of  a  given  quantity  of  power  may  also 
be  decreased.  The  charge  made  by  cables  has  been  explained  as  being 
measured  by  the  formula — 

TT2 

W  =  EC  =  C2E  =  ^ 
±c 

From  the  second  of  these,  C2E,  it  will  be  seen  that  the  charge  made 
by  the  cables  varies  directly  as  their  resistance,  and  as  the  square  of 
the  current  they  transmit,  so  that  any  reduction  in  the  current  reduces 
the  charge  for  the  same  resistance  in  the  ratio  of  its  square.  It  will 
also  be  seen  that  with  a  given  resistance  the  charge  varies  as  the 


236  ELECTRICITY   IN   MINING 

square  of  the  pressure,  the  pressure  in  this  case  meaning  that  which  is 
used  up  in  driving  the  current  through  the  cables.  Further,  from  the 
formula  E  =  CK,  it  is  evident  that  with  a  given  resistance,  the  smaller 
the  current,  the  smaller  the  pressure  required  to  drive  it  through. 
From  all  these  considerations  it  follows  that  doubling  the  pressure 
not  only  halves  the  current,  but  it  allows  the  cables  employed  to  be 
made  of  one  quarter  the  sectional  area,  or  a  quarter  the  weight  for  a 
given  length.  The  charge  for  the  passage  of  the  current  through  any 
resistance  is  halved,  and  doubling  the  pressure  gives  double  the 
available  pressure  for  use,  with  a  given  percentage  of  loss ;  hence  the 
above  saving.  From  this  it  will  be  seen  what  a  valuable  instrument 
is  placed  in  the  hands  of  the  engineer  for  distributing  power  with 
small  outlay,  providing  that  he  can  increase  the  pressure  as  much  as 
he  requires.  Another  point  had  perhaps  better  be  mentioned  here — 
the  effect  of  distance  upon  the  size  of  the  cables  for  the  transmission 
of  a  given  power.  It  was  explained  in  Chapter  I.  that  the  resistance 
of  any  conductor  of  a  given  sectional  area  varies  directly  as  its 
length.  It  follows,  therefore,  since  the  charge  both  upon  the  initial 
pressure  generated,  and  upon  the  power  delivered  to  the  cables, 
depends  directly  upon  the  resistance  of  the  cables,  that  the  charge 
will  increase  directly  as  the  length  of  the  cables,  unless  the  sectional 
area  of  the  conductor  is  increased  in  the  same  proportion  as  its  length 
is  increased.  That  is  to  say,  if  power  is  required  at  a  distance  of  two 
miles  from  the  generator,  the  cables  to  transmit  it  with  a  given  loss 
must  be  twice  the  sectional  area,  and  therefore  four  times  the  weight 
of  the  cables  required  to  transmit  the  same  power  to  a  distance  of 
one  mile.  Hence  the  importance  of  being  able  to  increase  the 
pressure  with  increased  distance,  and  increased  work  at  a  distance 
will  be  appreciated.  In  coal  mines,  and  in  metalliferous  mines,  the 
two  quantities,  distance  and  work  to  be  done  at  a  distance,  are  con- 
stantly increasing.  In  nearly  all  cases  as  the  mine  develops,  the 
distance  over  which  the  mineral  has  to  be  hauled,  and  often  that 
through  which  the  water  has  to  be  pumped,  increases,  while  the 
power  for  coal-cutting  machines,  drilling  machines,  etc.,  increases, 
and  has  to  be  delivered  at  greater  distances.  To  meet  the  increased 
cost,  increased  output  is  resorted  to,  and  this  means  that  increased 
work  has  to  be  done  from  the  increasing  distance,  leading  again  to 
the  necessity  of  high  pressures,  if  economy  is  to  be  realized. 


The  Two-wire  System 

The  simplest  of  all  arrangements  for  distributing  current  is  that 
known  as  the  two-wire  system,  as  shown  in  Fig.  99.  It  can  be  used 
with  continuous  current  machines,  and  with  single-phase  alternating 


DISTRIBUTION   OF  POWER   BY   ELECTRICITY     237 

current  machines,  though  the  latter  are  practically  barred  out  for  use 
in  mines  for  the  present.  With  this  arrangement  two  cables  are  led 
from  each  generator  to  the  main  switchboard,  as  will  be  explained,  and 
two  cables  are  led  from  the  switchboard  to  each  district,  or  each  part 
of  the  mine  that  is  to  be  supplied.  Thus  two  cables  would  be  led 
out  for  the  supply  of  the  surface  motors,  if  they  were  all  on  one  side 
of  the  generator  house,  or  more  than  two  sets  if  the  generator  house 
was  in  the  middle,  and  the  power  required  by  motors  lay  round  it. 
If  there  is  more  than  one  seam  worked,  a  pair  of  cables  are,  or  should 
be,  carried  from  the  main  switchboard  to  each  seam.  If  it  be  pre- 
ferred, one  pair  of  cables  may  be  taken  right  to  the  bottom  seam,  and 
branch  cables  attached  to  each  seam,  but  if  this  plan  is  adopted,  the 
cables  must  be  taken  well  in  out  of  the  shaft  to  a  dry  place  before 
tapping.  It  is  also  a  feasible  and  practical  arrangement  to  take  a 
cable  of  sufficient  size  to  supply  current  for  the  two  or  more  seams 
to  the  upper  seam,  and  to  carry  smaller  cables  from  the  upper  seam  to 


LAMP 


ARC 


FIG.  99.— Diagram  showing  Two-wire  System  of  Distribution. 

the  seams  below,  the  different  sections  of  the  cables  being  made  in  com- 
plete lengths.  With  metalliferous  mines,  where  a  number  of  levels  are 
worked  from  the  same  shaft,  and  where  there  is  a  great  deal  of  water 
in  the  shafts,  and  often  on  the  levels,  it  is  a  question  for  the  engineer 
in  charge  whether  he  will  run  a  pair  of  cables  for  each  level,  or 
whether  fee  will  work  on  what  is  known  as  the  tree  system,  taking 
large  cables  to  the  upper  level,  and  gradually  tapering  off  as  the 
mine  descends.  Both  arrangements  have  their  advantages,  and  if 
there  is  a  shortness  of  room  in  the  mine  shaft,  either  in  a  colliery  or  a 
metalliferous  mine,  the  engineer  may  be  obliged  to  take  only  one  pair 
of  cables  down.  Whatever  the  arrangement  may  be  that  is  made  for 
supplying  the  different  levels,  or  the  different  seams,  the  cables  are, 
or  should  be  taken,  to  a  switchboard,  as  will  be  explained,  at  each 
seam  or  level,  and  from  there  two  cables  should  be  taken  along  the 
roads  to  distributing  points,  where  again  fuseboards,  or  switch- 
boards, or  disconnecting  arrangements  of  some  kind  should  be  made, 
and  from  there  pairs  of  cables  carried  into  each  district  to  be  supplied, 


238  ELECTRICITY   IN   MINING 

and  so  on.  As  explained,  the  pressure  at  each  point  in  the  distribu- 
tion service  will  be  the  generator  pressure,  less  the  charge  made  for 
the  passage  of  the  current  through  the  resistance  between  it  and  the 
point  in  question.  In  the  author's  opinion,  this,  the  simple  two-wire 
system,  is  by  far  the  best,  where  continuous  currents  are  employed. 
The  two-wire  system  can  be  employed  for  practically  any  pressure ; 
for  100  or  110  volts  for  lighting  service,  which,  as  explained  in 
Chapter  III.,  may  be  provided  by  a  motor  generator;  for  200,  or 
220,  440,  500,  550,  and  600,  these  being  the  limits  to  which  con- 
tinuous currents  have  been  applied  in  mining  work.  The  two-wire 
system  may  also  be  employed  with  high  tension  single-phase  working, 
where  that  is  used,  as  say,  for  lighting,  but  with  the  aid  of 
transformers. 


The  Three=wire  System 

The  three-wire  system  the  author  does  not  recommend  for  mining 
work,  because,  in  his  opinion,  it  leads  to  complications  that  are  better 
avoided ;  but,  as  he  understands  that  it  has  been  employed  in  certain 
mines,  he  thinks  it  wise  to  give  a  description.  It  is  intended  to  give 
the  advantage  of  double  the  pressure  in  the  size  of  the  cables,  or 
nearly  so,  in  a  lighting  service  with  lamps  made  for  only  half  the 
pressure.  Thus,  when  incandescent  lamps  were  only  made  for  100 
•and  110  volts,  three-wire  systems  were  worked  at  200  and  220  volts. 
Now  that  lamps  are  made  for  as  high  as  260  volts,  three-wire 
systems  are  worked  at  from  400  up  to  520  volts.  With  the  early 
form  of  the  three-wire  system,  two  generators  of  the  lamp  voltage, 
100  or  110  in  the  early  days,  200  to  260  volts  now,  are  connected  in 
series,  the  positive  terminal  of  one  dynamo,  the  negative  terminal  of 
the  other,  being  connected  to  the  two  main  distributing  cables,  these 
cables  being  termed  "  outers."  The  junction  between  the  two  dyna- 
mos is  connected  to  a  middle  wire,  or  cable,  called  the  "  neutral,"  and 
this  cable  is  made  very  much  smaller  than  either  of  the  others.  The 
outer  cables  are  made  of  the  size  they  would  have  had  if  the  Combined 
pressure  of  the  two  generators  had  been  employed  in  the  ordinary 
way,  and  the  neutral  cable  is  made  about  half  the  size  of  one  of  the 
outers,  so  that  a  considerable  saving  in  copper  is  effected.  Lamps  are 
connected  between  the  positive  outer  and  the  neutral,  and  between 
the  negative  outer  and  the  neutral,  and  when  there  are  an  equal 
number  of  lamps,  or  an  equal  current  in  each  branch,  current  only 
passes  through  that  portion  of  the  neutral  wire  connecting  the  batches 
of  lamps  together,  the  current  passing  through  the  lamps  connected 
to  the  positive  outer,  then  through  the  lamps  connected  to  the 
negative  outer,  and  through  the  outer  to  the  machines,  the  neutral 
wire  merely  acting  as  a  connection  between  the  negative  terminals  of 


DISTRIBUTION    OF   POWER   BY   ELECTRICITY     239 

individual  lamps  in  the  negative  section,  and  the  positive  terminals  of 
individual  lamps  in  the  negative  section.  When  there  is  more  current 
passing  either  in  the  positive  branch  or  the  negative  branch,  the 
difference  in  the  current  passes  through  the  neutral  wire  to  the  joint 
terminal  of  the  two  machines.  Fig.  100  shows  the  connections  for 
this.  This  was  the  earliest  arrangement  of  the  three-wire  system, 
and  it  was  developed  in  the  case  of  Manchester  into  a  five- wire 
system,  with  four  generators  connected  in  series.  The  five-wire 
system  has  been  discontinued,  and  the  later  practice  is  to  have  only 
one  generator  furnishing  the  full  pressure,  400  to  520  volts,  as  may 
be  arranged,  the  terminals  of  the  generator  being  connected  to  the 
outers,  and  the  neutral  wire  having  no  connection  except  to  earth 
at  the  generating  station,  and  to  the  lamps.  This  arrangement 


O 


.LAMP 


FIG.  100.  -Diagram  of  the  Connections  for  distributing  on  the  Three-wire 
System  with  two  Generators.  Motors  are  frequently  connected  across 
the  Outers,  Lamps  always  between  one  Outer  and  the  Neutral  Wire. 

necessitates  some  provision  for  transferring  the  work  from  one  branch 
to  the  other  when  more  current  is  being  taken  by  one  branch  than 
the  other,  and  for  this  purpose  the  balancers  described  in  Chapter 
IV.  are  employed.  As  explained  in  that  chapter,  balancers  con- 
sist of  two  identically  similar  machines  having  the  axles  of  their 
armatures  mechanically  connected,  and  the  machines  themselves 
being  usually  fixed  on  one  bedplate,  and  they  may  be  arranged  to  be 
driven  either  automatically  by  the  service  itself,  so  that  there  is  an 
automatic  transfer  of  energy  from  the  side  doing  less  work  to  that 
doing  the  larger  amount  of  work,  or  they  may  be  driven  by  an  engine. 
In  either  case  both  machines  are  shunt- wound,  and  have  resistances 
connected  in  the  circuits  of  their  field  magnet  coils  that  can  be 
adjusted  at  the  main  switchboard,  or  at  an  auxiliary  switchboard  in 


240 


ELECTRICITY   IN   MINING 


special  cases.  However  the  balancers  may  be  driven,  the  armature 
of  one  of  the  machines  is  connected  across  each  branch,  as  shown 
in  Fig.  101,  which  represents  a  steam-driven  balancer,  and  when 
they  are  not  driven  by  a  steam  engine,  the  ends  of  their  field  coils 
are  connected  to  the  opposite  branch  to  that  to  which  their  arma 

POSITIVE:  OUTER 


NEUTRAL  WIRE  1 


i  LAMP 


FIG.  101. — Diagram  of  Connections  of  a  Steam-driven  Balancer. 

tares  are  connected.  The  field  coils  of  the  half  of  the  balancer 
whose  armature  is  connected  in  the  positive  branch  are  connected 
to  the  negative  outer  and  the  neutral,  as  shown  in  Fig.  102.  The 
field  coils  of  the  other  half  of  the  balancer  are  connected  to  the 
positive  outer  and  the  neutral.  When  one  branch  takes  more  current 
than  the  other,  the  pressure  between  the  outer  of  that  branch  and 

POSITIVE  OUTER  . 


NEGATIVE  OUTER 

FIG.  102. — Diagram  of  the  Connections  of  a  Motor-driven  Balancer.  The 
Field  Coils  of  each  Half  of  the  Balancer,  it  will  be  seen,  take  Current 
from  the  opposite  Half  of  the  System. 

the  neutral  is  lowered  in  consequence  of  the  increased  current. 
As  the  half  of  the  balancer  whose  armature  is  not  connected  to 
this  branch  takes  current  from  it,  the  current  passing  in  its  field 
coils  is  reduced,  and  this  leads,  as  will  be  explained  in  Chapter 
VI.,  to  its  running  as  a  motor  at  a  higher  speed  than  it  was  pre- 
viously running  at,  the  two  machines  running  simply  as  motors,  but 


C   o    c  O 

<D    QJ    t>  CD 

&§§  *! 

2  S'3  ^ 

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1 

5  r-     Cg     O    ^ 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY    241 

doing  no  useful  work  when  the  balance  is  even.  The  machine  in 
the  other  branch,  running  faster  as  a  motor,  drives  the  machine  in  the 
overloaded  branch  as  a  generator,  also  at  a  higher  speed  than  when  the 
balance  was  even,  causing  it  to  create  an  increased  pressure,  and  thus 
to  furnish  the  increased  current  required  by  that  branch.  When  the 
balancers  are  steam  driven,  each  is  connected  in  its  own  branch,  each 
has  its  own  adjustable  resistance  in  the  circuit  of  its  field  coils.  The 
two  are  driven  at  the  same  speed,  but  when  the  engineer-in-charge 
observes  that  one  branch  is  taking  more  current  than  the  other,  he 
increases  the  excitation  of  that  half  of  the  balancer,  thereby  increasing 
its  pressure,  and  providing  the  additional  current  required.  Modern 
practice  tends  towards  the  steam-driven  balancer ;  it  is  more  under 
control,  and  it  can  be  employed,  if  desired,  to  take  the  place  of  the 
generator  on  very  light  loads.  It  is  a  common  practice  with  three- 
wire  distribution,  with,  say,  from  400  to  520  volts  between  the  outers, 
to  supply  motors  with  current  from  the  outers,  and  lights  from  each  of 
the  branches,  the  lamps  being  distributed  as  evenly  as  possible,  so 
that  there  shall  be  the  same  number  of  lamps  burning  as  frequently 
as  possible  in  each  branch. 


The  Use  of  the  Accumulator  as  a  Balancer 

Accumulators  have  not  been  much  used  in  mining  work,  because 
they  are  somewhat  troublesome,  but  in  the  author's  opinion  they 
probably  will  be  as  time  goes  on,  and  he  thinks  it  wise  to  give  par- 
ticulars of  every  occasion  where  they  can  be  of  service.  They  have 
been  used  occasionally  as  balancers  on  three-wire  systems,  the  accu- 
mulator being  divided  into  two  batteries,  which  take  the  place  of  the 
motor  generator  balancer,  the  number  of  cells  in  each  branch  of 
the  service  being  either  controlled  by  hand  from  the  switchboard  as 
the  current  taken  from  either  side  increases  or  decreases,  or  being 
controlled  automatically.  The  accumulator  is,  of  course,  being 
charged  with  a  small  current  when  it  is  not  furnishing  any  current  to 
either  side,  and  the  regulation  may  be  performed  either  by  switching 
regulating  cells  in  and  out  in  each  branch,  or  by  fixing  a  resistance  in 
each  branch  in  series  with  the  accumulator,  and  switching  a  portion 
of  it  in  and  out. 

Distribution  by  Two  and  Three  Phase  Currents 

Two  and  three  phase  currents  may  be  employed  for  distributing 
current  for  light  and  power,  either  with  low,  medium,  high,  or  extra 
high  tensions.  Pressures  are  looked  upon  by  the  Home  Office  as 
follows: — Up  to  250  volts  are  considered  low  pressures;  between 

R 


242  ELECTRICITY  IN   MINING 

250  and  650  volts,  medium  pressures ;  between  650  and  3000  volts, 
high  pressures;  and  above  3000,  extra  high  pressures.  Two-phase 
currents  are  very  little  employed  in  mines,  but  three-phase  are, 
and  are  being  more  and  more  employed.  All  that  is  mentioned 
about  three-phase  currents  applies  to  two-phase,  with  the  proviso 
that  two-phase  currents  require  four  wires,  except  with  the  special 
arrangement  mentioned  below,  while  three-phase  require  only  three 
wires,  also  except  in  the  case  of  the  special  arrangements  men- 
tioned below.  As  explained  in  Chapter  I.,  the  two  currents  gene- 
rated by  a  two-phase  machine  are  provided  with  their  own  complete 
circuits  in  the  machine  and  outside  of  it,  the  two  circuits  being 
represented  by  two  distinct  pairs  of  cables,  connecting  the  machine 
with  the  lamps  or  motors.  It  will  be  seen  that  there  is  a  certain 
disadvantage  in  this  matter  with  two-phase,  since  it  is  possible 
to  make  connections  between  cables  belonging  to  wrong  phases, 
and  it  is  not  easy  within  the  mine,  unless  the  cables  are  very  care- 
fully distinguished  by  being  braided  in  different  colours,  and  then 
the  colours  are  apt  to  be  extinguished  by  the  all-pervading  black  of 
coal-dust,  or  the  dull  grey  of  the  metalliferous  mine,  while  with  the 
three-phase  service  there  can  be  no  mistake  whatever.  The  arrange- 
ment of  the  cables  of  a  two-phase  service  may  be  modified  by  making 
one  cable  the  common  return  for  the  two  phases,  three  cables  only 
then  being  required ;  but  there  is  still  the  same  danger  of  connecting 
lamps,  say,  between  the  cables  belonging  to  the  two  phases,  unless 
some  special  arrangement  is  made  to  prevent  it.  With  three-phase 
currents,  as  explained,  there  are  three  cables  connected  to  the  three 
terminals  of  the  machine,  lamps  being  connected  between  either 
two  cables,  and  finding  between  them  the  pressure  the  service  is 
delivering.  There  is  a  modification  of  the  three-phase  service, 
however,  that  is  sometimes  advocated,  mainly  in  connection  with  the 
star-connected  machine.  In  this  arrangement  the  neutral  point  of 
the  armature  conductors  is  connected  to  one  cable,  and  may  be 
arranged  for  incandescent  lamps  to  be  connected  between  the  three 
ordinary  service  cables  and  the  neutral  cable,  the  three  ordinary 
service  cables  being  connected  to  motors.  This  arrangement  is 
principally  of  service  where  the  three-phase  system  is  worked  at  a 
pressure  of  440  volts  between  the  ordinary  cables,  the  pressure 
between  any  of  these  and  the  neutral  cable  then  being  250  volts, 
which  is  nearly  the  present  limiting  pressure  for  incandescent  lamps. 
In  the  author's  opinion,  the  arrangement  is  not  a  wise  one.  It  is 
only  when  the  special  voltage  named  is  used  that  it  assumes  any 
convenience  whatever,  and  it  introduces  a  fourth  wire  into  the  system 
with  the  complications  that  it  unfortunately  brings.  In  his  opinion, 
everything  about  a  mine  should  be  kept  as  simple  as  it  is  possible. 
Three-phase  currents  are  being  used  in  mines  for  200  to  220  volts, 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     243 

also  for  440,  500,  550,  600,  and  up  to  3000  volts,  the  service  in  the 
case  of  these  pressures  being  direct.  That  is  to  say,  the  full  pressure, 
whatever  it  may  be,  is  delivered  by  the  generators  to  the  switchboard, 
and  from  the  switchboard  to  the  distributing  cables,  or  feeders,  and 
from  them  to  the  lamps  and  motors.  Motors  of  up  to  50  H.P.  can 
be  worked  with  pressures  below  650  volts,  but  above  that  power  the 
windings  and  the  insulation  become  difficult  with  the  low  pressures, 
and  therefore  higher  pressures  are  employed.  For  the  lighting  service, 
as  already  explained,  arc  lamps  or  incandescent  lamps  may  be  con- 
nected in  series  between  any  two  of  the  three  cables  of  the  three- 
phase  system,  and  between  each  of  the  pairs  of  cables  of  the  two- 
phase  system,  or  between  the  cables  and  the  neutral ;  but  the  author's 
view  is,  that  it  is  wiser  to  employ  a  motor  generator,  its  motor  taking 
current  from  the  three-phase  service,  and  its  generator  delivering  con- 
tinuous currents  at  100  volts  or  so  for  the  lighting  service.  For  the 
power  service,  the  three  or  the  four  wires  of  the  three-phase  and  two- 
phase  systems  are  taken  direct  to  the  motors,  with  medium  and  high 
pressures,  through  the  starting  switches,  etc.,  as  will  be  explained  in 
Chapter  VI. 

Distribution  by  Two  and  Three  Phase  Currents 
at  High  Tension  and  at  Extra  High  Tension 

Where  high  tensions  or  extra  high  tensions  are  employed  with 
two  and  three  phase  currents,  stationary  transformers  are  employed. 
The  current  may  be  generated  at  the  full  pressure,  say,  at  3000  volts, 


.  D 

.     E 

1 

THREE  Ptose 


PIG.  103. — Diagram  of  Connections  for  Three-phase  High  Tension  Distribution 
with  one  Transformer. 

or  it  may  be  generated  at  a  lower  pressure,  say,  500  or  550  volts,  and 
transformed  up  to  3000  volts.  In  the  case  of  extra  high  tensions  the 
currents  are  always  generated  at  a  lower  pressure  than  that  which  is 
to  be  employed  for  transmission,  and  they  are  transformed  up.  As 


244 


ELECTRICITY   IN    MINING 


explained  in  describing  stationary  transformers  in  Chapter  IV.,  the 
transformer  may  be  employed  to  increase  the  pressure  delivered  by 
the  generator,  or  to  decrease  it,  or  for  both.  Thus,  where  power  is 
transmitted  over  long  distances,  as  where  a  number  of  mines  are 
taking  power  from  a  waterfall  at  some  distance,  it  is  usual  to  gene- 
rate the  currents  at  500  volts,  or  thereabouts,  to  transform  them  up 
by  stationary  transformers  at  the  generating  station  to  the  10,000  or 
20,000  volts,  or  whatever  the  line  pressure  may  be,  to  transmit  the 
power  by  their  aid  through  the  wires  leading  to  the  points  of  con- 
sumption at  these  pressures,  and  they  are  there  transformed  down  to  the 
pressures  at  which  the  currents  are  to  be  employed.  The  transforma- 
tion is  very  often  accomplished  by  two  sets  of  transformers.  Thus, 
if  the  currents  are  generated  at,  say,  500  volts,  the  first  set  of 
transformers  may  increase  the  pressure  to  2000  volts,  and  the 
secondary  currents  from  these  transformers  may  be  taken  as  the 


Fia.  104. — Diagram  of  Connections  for  Three-phase  Extra  High  Tension 
Distribution  with  Step  Up  and  Step  Down  Transformers. 

primary  currents  of  a  second  set  of  transformers,  in  which  the 
pressure  is  increased  to  10,000  or  20,000,  or  whatever  the  line 
pressure  determined  upon  may  be.  And  the  same  process  may  take 
place  at  the  consumer's  end.  The  line  pressure  may  be  reduced  first 
to  2000  or  3000  volts,  and  any  motors  that  are  working  at  that 
pressure  supplied  from  the  secondary  coils  of  this  first  set  of  trans- 
formers, and  a  portion  of  the  current  taken  to  a  second  batch  of 
transformers  where  it  is  reduced  to  500,  or  whatever  the  working- 
pressure  of  the  other  motors  may  be,  and  this  may  be  carried  farther 
by  transforming  a  small  portion  of  the  current  by  separate  transformers, 
specially  to  100  or  110  volts  for  the  lighting  service.  As  explained 
also,  the  rotary  convertor  is  a  transformer  as  well  as  a  convertor,  and 
may  be  employed  for  the  purpose,  within  the  limits  of  its  own 
capacity.  Figs.  103  and  104  are  diagrams  of  connections  for  high 
tension  and  extra  high  tension  three-phase  distribution. 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     245 


The  Main  Switchboard 

By  the  Home  Office  Kegulations  for  the  use  of  electricity  in  coal 
mines,  it  is  necessary  that  a  main  switchboard  shall  be  provided.  It 
is  also  advisable  in  every  case,  whether  required  by  law  or  not.  The 
main  switchboard  is  the  clearing-house  of  the  generating  and  dis- 
tributing systems.  The  current  from  all  the  generators  is  brought  to 
the  main  switchboard,  the  current  for  all  the  motors,  lamps,  etc.,  is 
taken  from  the  main  switchboard.  The  switchboard  itself  consists  of 
slabs,  either  of  marble  or  enamelled  slate,  the  marble,  if  chosen,  being 
free  from  metallic  veins,  the  slabs  being  mounted  on  a  steel  framing, 
fixed  vertically.  The  slabs  are  known  as  panels,  and  there  should  be 
a  panel  for  each  generator,  a  panel  for  each  feeder  or  distributor  set  of 
cables,  and  panels  for  each  auxiliary  apparatus,  such  as  boosters, 
accumulator  switch  gear,  etc.  On  each  generator  panel  there  should 
be  an  ampere  meter,  showing  at  any  instant  the  current  the  machine 
is  delivering,  a  volt  meter,  showing  the  pressure  at  which  it  is  being 
delivered  at  the  switchboard,  switches  to  disconnect  the  generator 
completely  from  the  switchboard,  one  or  more  circuit  breakers  as  they 
are  called,  to  disconnect  the  generator  automatically  from  the  switch- 
board, fuses  in  each  lead  also,  to  disconnect  it  automatically  from  the 
switchboard,  and  there  is  usually  a  field  current  regulator,  generally  a 
wheel  fixed  in  front  of  the  board,  which  turns  an  arm  over  a  succession 
of  contacts  arranged,  the  successive  contacts  cutting  in  or  out  succes- 
sive lengths  of  a  resistance  employed  to  regulate  the  strength  of  the 
current  passing  in  the  field  coils  of  the  generator.  On  the  generator 
panel  also  is  often  carried  a  meter,  showing  the  number  of  units  the 
generator  has  furnished  during  any  period.  It  also  sometimes  carries 
recording  ampere  and  volt  meters,  designed  for  the  same  purpose, 
these  giving  a  record  upon  a  chart,  similar  to  that  of  a  barograph,  of 
the  variations  of  pressure  and  current  furnished  by  the  generator 
during  the  twenty-four  hours.  Each  feeder  or  distributor  panel 
should  carry  an  ampere  meter,  switches,  circuit  breakers,  and  fuses, 
for  disconnecting  it  from  the  switchboard.  It  also  sometimes  carries 
meters  showing  the  number  of  units  delivered  to  each  feeder  during 
the  twenty-four  hours,  and  sometimes  recording  ampere  meters.  It 
is  also  sometimes  arranged  to  have  pilot  volt-meter  wires  at  certain 
distributing  or  feeding  points,  as,  say,  the  pit  bottom,  the  distributing 
point  in-bye,  etc.,  the  pilot  wires  being  small  signal  wires,  and  in  this 
case  being  connected  with  a  volt  meter  fixed  on  the  feeder  panel, 
showing  the  attendant  the  pressure  at  any  instant  at  the  distributing 
points.  A  recording  volt  meter  is  also  sometimes  fixed  on  the  feeder 
panel  of  the  switchboard,  recording  the  variations  of  pressure  at  this 
point. 


246  ELECTRICITY  IN   MINING 

The  accumulator  booster  panels  will  carry  ampere  and  volt  meters, 
switches,  circuit  breakers,  fuses,  rheostats,  etc.,  enabling  the  attendant 
to  completely  control  the  working  of  this  apparatus  at  the  switch- 
board. 

Where  three-phase  currents  are  employed,  and  the  bus  bar 
system,  explained  below,  is  also  employed,  synchronizing  apparatus 
is  necessary.  This  is  carried  on  a  panel  by  itself.  By  synchronizing 
is  meant,  arranging  that  the  pressures  and  currents  generated  by  the 
machine  that  is  about  to  be  connected  to  the  service  are  exactly  the 
same,  at  any  instant,  as  those  already  in  service,  the  currents  and 
pressures  rising  and  falling  exactly  in  unison  with  those  already 
passing  in  the  system,  and  for  this  purpose  it  is  necessary  to  have 
some  apparatus  which  will  show  when  the  two  are  in  unison. 
Synchronizing  apparatus  is  described  on  p.  249. 

There  are  also  switches  arranged  for  connecting  the  feeders  to  the 
"  bus  bars,"  and  for  connecting  each  generator  to  them.  Plates  12 A 
and  13 A  show  main  switchboards  suitable  for  mining  work,  and 
Plate  12B  shows  the  back  of  the  board  shown  in  Plate  12 A,  with  the 
cables,  etc. 


The  Parallel  or  Bus  Bar  System 

There  are  two  methods  of  arranging  the  connections  between  the 
generators  and  the  feeders  or  distributing  cables,  known  respectively 
as  the  "  parallel "  or  "  bus  bar "  system  and  the  "  independent " 
system.  In  generating  stations  for  town  supply,  and  for  the  distri- 
bution of  power  in  the  counties,  the  bus  bar  system  is  almost 
universally  employed.  It  has  also  been  adopted  in  many  of  the 
collieries  where  power  stations  have  been  laid  down  for  groups  of 
mines.  In  the  bus  bar  system  there  are  two  or  three  substantial 
copper  bars  fixed  on  the  main  switchboard,  usually  behind  the  board, 
though  some  firms  prefer  to  fix  them  above  the  board.  The  generators 
which  are  supplying  the  service  must  deliver  current  at  the  bus  bars 
at  exactly  the  same  pressure.  If  the  pressure  delivered  by  any  gene- 
rator at  the  bus  bars  falls  below  that  delivered  by  the  other  generators, 
it  not  only  cannot  deliver  current,  but  its  own  coils  become  paths  for 
the  current  supplied  by  the  other  generators,  and  this  was  the  author's 
great  objection  to  the  use  of  the  bus  bar  system  in  private  works. 
Where,  however,  the  generating  station  is  of  sufficient  size  to  warrant 
keeping  a  sufficiently  skilled  attendant  at  the  switchboard,  the  danger 
practically  disappears ;  and,  on  the  other  hand,  the  bus  bar  system 
enables  the  engineer-in-charge  to  conveniently  distribute  the  load 
between  the  machines  in  service  as  he  pleases.  Figs.  105  and 
106  show  the  connections  for  shunt- wound  and  separately  excited 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     247 


machines,   when    connected    to  the  bus    bars.      As    explained    in 
Chapter  IV.,  the  initial  pressure  created  by  the  armature   of  any 

-  BUSBAR 


+  BUSBAR. 


•p  oo  Oopo  t^i 
FIE.UO  Coius 


FIELD  Coius 


FIG.  105. — Diagram  of  Connections  of  two  Shunt-wound  Continuous  Current 
Generators  to  a  pair  of  Bus  Bars. 

generator  is  subject  to  a  charge  for  the  passage  of  the  current  through 
its  coils  to  the  brushes,  and  this  lowers  the  actual  pressure  delivered 
by  the  machine,  exactly  in  proportion  to  the  product  of  the  current 


BUSBAR 


PIG.  106. — Diagram  of  Connections  of  two  Separately  Excited  Machines 
to  a  pair  of  Bus  Bars. 

passing,  and  the  resistance  of  the  armature.  When  no  current  is 
passing,  the  pressure  at  the  brushes  is  the  initial  pressure  created  by 
the  armature,  and  it  becomes  steadily  less  as  the  current  passing 


248 


ELECTRICITY   IN   MINING 


through  the  armature  increases.  The  engineer-in-charge  has  at  his 
command  two  variable  quantities,  with  both  continuous  and  alter- 
nating current  machines,  by  which  he  can  alter  the  pressure  at  will, 
viz.  the  speed  at  which  a  machine  is  running,  and  the  exciting  current 
passing  through  the  coils  of  its  field  magnets.  In  practice  the  speed 
is  not  much  altered  with  continuous  current  machines,  and  not  to  a 
large  extent  with  alternating  current  machines,  the  pressure  being 
raised  or  lowered  by  switching  out,  or  in,  resistance  in  the  field  coils, 
by  the  rheostat  on  the  switchboard.  When  the  machine,  whether 
continuous  current  or  alternating,  is  brought  into  service,  if  the  pres- 
sure it  is  delivering  at  its  terminals,  when  no  current  is  passing,  is 
exactly  the  same  as  the  pressure  existing  between  the  bus  bars  at  the 

BUSBAR     N°  I 


,    BUSBAR     r\°  2  ftnAse 

BUSBAR  Np  3  PHASE 

M 

<v( 

n 

i                              «« 

c 

/n°3                             P«i£i,     ! 
/"""                                       \ 

/*" 

ARMATURE                                  ARMATURE 

FIG.  107. — Diagram  of  Connections  of  two  Three-phase  Generators  connected  to 

three  Bus  Bars. 

instant,  it  will  furnish  no  current  to  the  outer  service,  and,  on  the 
other  hand,  its  coils  will  not  provide  a  path  for  current  from  the  other 
machines.  If  its  pressure  is  above  that  of  the  bus  bars,  it  will  imme- 
diately furnish  current  to  the  system,  until  the  charge  made  upon  its 
initial  pressure  for  the  passage  of  the  current  brings  its  pressure  at 
the  bus  bars  down  to  the  pressure  of  the  other  machines  delivering. 
Hence,  by  increasing  the  excitation  of  any  given  generator,  the  pro- 
portion of  the  load  it  takes  is  increased,  and  by  decreasing  the 
excitation,  the  proportion  is  decreased,  providing  that  the  steam 
furnished  to  the  engine  driving  the  generator  is  proportioned  to  the 
work  the  machine  is  being  called  upon  to  perform.  Hence  it  will 
be  seen  that,  providing  the  engineer-in-charge  and  the  switchboard 
a  ttendant  understand  the  matter,  the  distribution  of  the  load  between 


PLATK  16A. — Westinghouse  Iron  Distri- 
buting Boxes,  for  use  in  Mines. 


PLATE  16B.  —  Westinghouse  High 
Tension,  Gas  Proof  Switch  for  use 
in  Mines.  The  Door  cannot  be  opened 
if  the  Switch  is  closed. 


PLATE  16c. — Ferranti  Double  Pole,  110 
Voltage  Circuit  Breaker,  with  Cover 
removed.  The  Circuit  is  remade  by 
the  Handle  shown  below. 


PLATE  16D. — Ferranti  Double  Pole  Over- 
load and  Reverse  Currents  Carbon. 
Circuit  Breaker  with  Circuit  Oven. 


[To  face  p.  248. 


DISTRIBUTION    OF   POWER   BY   ELECTRICITY     249 

the  generators  in  service  is  a  very  simple  affair.  For  instance,  suppose 
the  station  to  have  been  running  on  light  load,  with  one  generator  and 
its  accessories  furnishing  the  whole  of  the  current  required,  and  the 
heavier  load  to  be  gradually  coming  on.  The  engineer-in-charge  will 
run  up  a  second  unit  to  its  proper  speed,  arrange  its  exciting  current 
so  that  it  furnishes  either  the  same  or  a  slightly  higher  pressure  than 
that  of  the  bus  bars,  bring  it  to  synchronism,  if  the  service  is  alter- 
nating, switch  it  on  to  the  bus  bars,  and  then  gradually  increase  the 
pressure  it  is  generating,  also  increasing  the  supply  of  steam  or  gas  to 
its  engine,  till  it  has  taken  the  proportion  of  the  load  he  intends  it 
to.  If  the  station  is  run  with  a  small  machine  during  light  load,  and 
this  machine  is  allowed  to  rest  and  cool  during  the  time  of  heavy 
load,  he  will  gradually  relieve  the  light  load  machine  of  the  whole  of 
its  load,  bring  its  pressure  down  to  that  of  the  bus  bars,  and  then 
switch  it  off.  With  alternating  currents,  either  single,  two  or  three 
phase,  as  explained,  it  is  also  necessary  to  bring  the  incoming  machine 
into  synchronism,  and  this  is  done  by  the  apparatus  described  below. 
Fig.  107  shows  the  connections  for  three-phase  generators,  when  con- 
nected to  bus  bars. 

Synchronizing  Apparatus 

The  earliest  arrangement  of  synchronizing  apparatus  consisted  of 
two  incandescent  lamps  receiving  current  from  the  secondary  coils 
of  two  transformers,  the  primary  coils  of  which  were  connected,  one 
to  the  bus  bars,  and  the  other  to  the  incoming  machine,  the  two 
lamps  with  the  two  secondary  coils  being  connected  in  series.  When 
a  machine  was  to  be  put  into  service  it  was  run  up  to  speed,  its 
pressure  regulated,  and  then  the  synchronizing  apparatus  was  con- 
nected. When  the  currents  from  the  machine  coming  in,  and  the 
bus  bars,  were  in  synchronism,  both  lamps  would  glow  brightly,  and 
the  light  would  vary  as  the  machines  got  in  or  out  of  synchronism, 
the  changes  being  very  visible  in  the  lamps.  In  some  cases  one 
lamp  was  employed,  connected  to  the  two  secondary  coils.  When 
the  lamp  was  seen  to  be  burning  brightly  the  attendant  would  switch 
the  machine  in,  and  if  he  had  judged  rightly  the  incoming  machine 
would  then  take  a  small  portion  of  the  load,  which  could  be  increased 
by  increasing  the  excitation  of  the  field  magnets  and  the  steam 
supplied.  If  he  made  a  mistake  when  synchronizing,  either  the 
incoming  machine,  or  those  in  service  would  receive  current  through 
their  coils,  and  the  synchronizing  current,  as  it  is  called,  would  tend 
to  pull  the  machines  into  synchronism.  The  synchronizing  current 
sometimes  strains  a  machine,  especially  if  it  is  a  heavy  current. 
Apparently  the  machines  themselves  dislike  a  synchronizing  cur- 
rent, as  in  some  cases  they  give  a  loud  screech.  The  later  forms  of 


250 


ELECTRICITY   IN   MINING 


apparatus  consist  of  dial  instruments  fixed  on  the  synchronizing 
panel,  carrying  needles  in  front  which  move  to  the  right  or  to  the 
left,  according  as  the  periodicity  of  the  incoming  machine  is  greater 
or  less  than  that  of  the  bus  bars.  The  synchronizers  or  synchroscopes, 
as  they  are  called,  consist  of  coils  of  wires  carrying  currents  taken 
from  the  bus  bars  and  from  the  incoming  machine,  and  the  arrange- 
ment is  very  similar  to  that  of  the  induction  motor.  When  the 
machines  are  out  of  synchronism  one  of  the  coils  tends  to  turn  upon 
its  axis,  it  being  movable  and  carrying  a  needle,  and  the  extent  to 
which  it  moves  shows  how  much  the  incoming  machine  is  out  of 
synchronism.  It  is  evident  that  the  lamp  system  and  the  dial 
synchronizer  can  be  used  together,  but  the  dial  synchronizer  is 
gradually  displacing  the  lamp  system,  as  being  better,  simpler,  and 
more  accurate.  In  America  the  lamp  system  was  worked  on  opposite 
lines  to  those  which  rule  in  this  country.  Synchronism  was  shown 
when  the  lamps  were  dark.  In  the  author's  opinion  this  is  hardly  a 
satisfactory  arrangement,  as  there  is  a  large  portion  of  the  pressure 
of  any  lamp  service  during  which  the  lamp  is  perfectly  dark. 


Paralleling  Compound  Continuous  Current 
Machines 

The  paralleling  of  shunt-wound  continuous  current  machines  is 
carried  out  as  shown  on  p.  247,  but  it  is  sometimes  an  advantage, 


BUSBAR 


COILS 


COILS 


BAR/ 


FIG.  108. — Diagram  of  Connections  of  two  Compound  Machines  connected 
to  two  Bus  Bars. 

as  explained,  to  employ  compound  machines,  as  where  it  is  convenient 
to  raise  the  pressure  at  a  given  distributing  point,  as  the  current 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     251 

delivered  from  that  point  increases,  and  it  then  becomes  not  quite 
such  a  simple  problem  to  connect  the  machines  in  parallel,  to 
ensure  that  each  shall  take  its  share  of  the  load  it  was  intended  to, 
and  that  the  current  in  the  series  coils  shall  not  vary  the  pressure 
delivered  by  its  own  machine  in  a  manner  it  was  not  intended  to. 
The  connections  to  the  bus  bars  are  arranged  thus :  the  positive 
brush  is  connected  to  the  positive  bus  bar,  and  the  end  of  the  series 
coil  forming  the  other  terminal  of  the  machine  is  connected  to  the 
negative  bus  bar,  and  what  is  called  an  "  equalizing  "  bar  is  fixed  on 
the  switchboard,  and  connections  are  made  to  the  equalizing  bar 
from  the  negative  brushes  of  all  the  machines  in  service.  The  object 
of  the  "  equalizing "  bar  is  the  same  as  that  of  the  equalizing 
connections  mentioned  in  describing  the  construction  of  multipolar 
continuous  current  generators,  viz.  to  maintain  the  pressures  between 
the  positive  and  negative  terminals  of  the  different  machines  at  the 
same  figure.  The  arrangement  does  not  interfere  with  the  distribution 
of  the  load,  it  merely  ensures  that  the  pressures  of  each  individual 
machine  shall  be  the  same.  The  connections  are  shown  in  Fig.  108. 


The  Independent  System 

In  the  independent  system,  which  the  author  strongly  advocated 
for  private  works,  and  which  he  still  advocates  in  those  cases  where 
the  plant  is  small,  and  is  left  in  charge  of  an  unskilled  man,  each 
generator  is  connected  directly  to  one  or  more  sets  of  feeder  cables, 
there  being  no  connection  between  the  generators  themselves  nor 
between  the  sets  of  feeder  cables,  except  when  two  or  more  are  con- 
nected to  the  same  generator.  The  working  arrangement  is  as  follows. 
During  light  load  the  one  generator  running  will  be  connected  to  all 
the  feeder  cables,  and  will  supply  all  the  current.  When  the  load 
commences  to  increase,  and  the  additional  units  are  to  be  brought 
into  service,  one  or  more  sets  of  cables  are  switched  over  from  the 
light  load  generator  to  one  or  other  of  the  new  generators,  as  they 
come  into  service.  When  the  load  decreases  again,  and  the  full  load 
generators  are  to  be  taken  out  of  service,  the  feeder  cables  are 
gradually  switched  from  each  of  the  units  on  to  the  light  load 
machine.  The  switch  arrangement  is  more  complicated  than  with 
the  bus  bar  system.  It  will  be  seen  that  in  the  case  of  the  bus  bar 
system  the  whole  arrangement  is  simplicity  itself.  Any  machine  can 
be.  withdrawn  from  service  by  throwing  its  switch  open,  and  any 
feeder  can  be  disconnected  from  the  system  by  throwing  its  switch 
open,  though  this  should  only  be  done  when  no  current  is  passing. 
Further,  except  in  case  of  accident,  the  whole  of  the  feeders  remain 
continuously  connected  to  the  bus  bars,  the  only  changes  that  are 


252  ELECTRICITY  IN   MINING 

made  being  in  connecting  or  disconnecting  successive  generators. 
With  the  independent  system,  however,  two  distinct  operations  have 
to  be  performed,  the  feeder  cables  have  to  be  disconnected  from 
the  machine  from  which  they  are  receiving  current  at  the  moment, 
and  connected  to  the  machine  from  which  they  are  now  to  receive 
current,  this  involving  a  distinct  break  in  the  service  and  a  wink  in 
the  lights,  though  this  may  not  be  serious.  The  arrangement  also 
involves,  sometimes,  some  complication  where  there  are  several 
machines,  and  a  great  deal  of  care  at  the  switchboard  to  avoid 
arcing. 

When  there  are  only  two  machines,  as  in  the  case  of  a  small 
mine,  the  arrangement  on  the  independent  system  is  very  simple. 
Each  feeder  has  a  "two-throw"  switch,  as  it  is  called,  arranged 
with  its  contact  bar  permanently  connected  to  one  feeder  cable,  with 
continuous  currents,  and  with  the  three  contact  bars  or  the  four 
contact  bars  connected  to  the  three  or  four  cables,  with  three  or  two 
phase  alternating  currents.  The  contact  bars  are  faced  by  two  sets  of 
contacts,  fixed  widely  apart,  connected  to  one  terminal  of  each  of  the 
machines,  the  other  terminals  of  all  the  machines  being  connected  to 
a  common  return,  to  which  also  the  return  cables  of  the  feeders  are 
also  connected.  When  it  is  desired  to  switch  over  a  given  feeder  from 
one  machine  to  the  other,  the  contact  bar  is  very  quickly  disconnected 
from  the  contacts  with  which  it  is  in  connection,  and  rapidly  pushed 
into  contact  with  the  other  set  of  contacts,  the  operation  only  occupy- 
ing a  few  seconds.  For  more  than  two  machines,  almost  the  only 
arrangement  possible  is  a  similar  one  to  that  which  was  explained  in 
connection  with  telephone  exchange  service.  One  set  of  terminals 
of  all  the  machines  and  one  set  of  the  distributors  are  connected 
together  with  continuous  current  machines,  and  the  other  terminals 
of  the  machines  are  connected  to  their  own  bars,  which  are  fixed 
either  horizontally  or  vertically,  the  two  sets  of  bars  having  holes  in 
them  where  they  cross,  and  connection  being  made  by  plugs  passing 
through  the  two.  The  connection  has  to  be  made  from  one  to  the 
other  with  this  system  very  quickly,  and  the  author  does  not  see 
how  it  is  possible  to  arrange  a  system  in  a  mine,  with  two  OP  three 
phase  currents,  since  all  switching  must  be  absolutely  instantaneous. 
The  independent  system,  however,  with  switching  on  these  lines  was 
carried  out  for  some  time  at  a  few  of  the  town  electricity  generating 
stations,  but  is  now  practically  displaced  everywhere  by  the  bus  bar 
system.  The  author's  advice  would  be  to  employ  the  independent 
system  where  the  plant  is  small,  and  a  skilled  attendant  cannot  be 
afforded,  but  to  employ  the  bus  bar  system  where  more  than  two 
machines  are  employed,  and  to  also  employ  an  attendant  of  sufficient 
skill  to  deal  with  it,  whatever  the  cost  may  be. 

The  arrangement  of  bars  crossing  each  other  at  right  angles,  for 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     253 

connecting  machines  and  feeders,  is  used  with  the  bus  bar  system, 
but  switching  there  only  takes  place  when  the  current  is  off,  other 
switches  having  been  opened  previously,  to  break  the  connection. 

Switchboard  Gear  for  High  Tensions  and 
Extra  High  Tensions 

For  high  tension,  and  more  particularly  for  extra  high  tensions, 
where  employed,  special  arrangements  are  necessary,  both  to  protect  the 
switchboard  attendants,  to  prevent  the  formation  of  arcs  between  the 
different  portions  of  the  switch  gear,  and  to  prevent  the  breaking  down 
of  the  insulation  between  different  portions  of  the  switching  apparatus, 
by  the  leakage  current  which  is  always  passing.  The  usual  arrange- 
ment employed  is,  the  switch  gear  for  each  generator,  and  for  each 
feeder  is  enclosed  in  a  separate  cell,  built  up  of  brickwork,  or  some 
similar  arrangement,  and  with  a  substantial  thickness  between  adjacent 
cells.  The  same  system  is  adopted  in  connection  with  the  cell  arrange- 
ment as  on  the  switchboard.  There  will  be  the  switch,  the  circuit 
breakers,  fuses,  transformers  where  required,  in  one  set  of  cells  which 
will  be  divided,  as  explained,  from  the  next  set  of  cells,  and  each  of 
the  cells  belonging  to  each  set  will  also  be  divided  from  each  other 
by  brickwork,  or  similar  arrangements.  The  switches  are  worked 
electro-magnetically  from  what  is  called  an  operating  board,  fixed 
usually  in  front  of,  but  a  little  distance  from,  the  switch  cells.  On  the 
operating  board  are  smaller  switches,  which  enable  the  switchboard 
attendant  to  operate  the  large  switches  in  their  cells.  The  main 
switches  are  usually  arranged  so  that  the  working  contacts  are  enclosed 
in  a  tank  of  oil,  similar  to  that  employed  for  the  immersion  of  trans- 
formers, and  a  common  arrangement  is,  two  vertical  rods,  representing 
the  two  fixed  contacts  of  the  ordinary  switch,  project  from  two 
insulators  above,  down  into  the  oil  in  the  tank,  where  they  are  faced 
by  a  contact  piece,  which  is  moved  by  a  third  vertical  rod  controlled 
from  above.  The  contact  rod  is  sometimes  moved  by  an  electric 
motor,  and  sometimes  by  a  solenoid.  The  B.  T.  H.  Co.  employ  a 
motor,  the  Westinghouse  Co.  a  solenoid.  In  either  case  the  motor  or 
the  solenoid  are  supplied  with  current  from  a  low-tension  service, 
controlled  by  a  switch  on  the  operating  switchboard,  and  when  the 
switchboard  attendant  closes  the  switch  on  the  operating  switch- 
board, a  current  passes  round  the  motor,  or  the  solenoid  coils,  causing 
them  to  move  the  contact  bar  into  connection  with  the  vertical  contact 
pieces.  When  the  switch  is  to  be  opened,  the  controlling  switch 
on  the  operating  board  is  opened,  and  springs,  weights,  or  other 
equivalent  mechanism  come  into  operation,  pushing  the  contact  bar 
away  from  the  fixed  contact  rods,  and  opening  the  circuit,  the  arc 


254  ELECTRICITY  IN   MINING 

which  is  formed  being  in  the  oil  in  the  tank,  and  being  quickly  extin- 
guished by  it.  This  arrangement  may  be  employed  for  any  pressures 
from  650  upwards,  but  it  is  usual  in  mining  work,  after  650  volts 
have  been  passed,  to  go  direct  to  2000  or  3000,  preferably  the  latter, 
as  it  is  the  limit  of  high  tension  working.  It  is  of  more  importance 
to  enclose  the  contact  arrangement  within  separate  cells,  as  described, 
as  the  pressure  increases.  Plates  14A  and  14B  show  a  Ferranti 
three-phase  10,000  volt  switch,  electrically  operated ;  and  Plates  14c 
and  14D,  and  17c,  Ferranti's  mining  and  high-tension  switches 
operated  by  hand;  and  Plate  1?D,  Messrs.  Keyrolle's  high  tension 
three-phase  hand  switch. 


Sub = station  Switchboards 

Switchboards  are  necessary  at  all  sub-stations,  that  is  to  say, 
wherever  the  current  is  received  for  distribution  from  the  main 
switchboard.  Where  the  mine  receives  current  from  a  power  station 
supplying  a  number  of  mines,  some  form  of  switchboard  is  necessary 
to  deal  with  it  on  its  arrival.  Figs.  109  and  110  show  the  connec- 
tions of  a  sub-station  switchboard  for  colliery  work,  taking  current 
at  6000  volts,  and  transforming  to  2000,  as  arranged  by  Messrs. 
Eeyrolle.  Also,  at  places  such  as  pit  bottoms,  distributing  points 
in-bye,  etc.,  which  may  be  termed  sub-stations,  switchboards  are 
necessary.  The  switchboards  for  dealing  with  current  received  at  any 
individual  mine,  from  the  main  generating  station  will  be  a  small 
replica  of  the  main  switchboard,  there  being  one  panel  corresponding 
to  the  generator  panels  of  the  main  switchboard,  for  the  current 
received  from  the  main  generating  station,  which  will  have  an  ampere 
meter,  a  volt  meter,  main  switch,  circuit  breakers  and  fuses,  with 
sometimes  meters  and  recording  volt  and  ampere  meters.  The  distri- 
butor or  feeder  panels  will  be  counterparts  of  the  distributor  and  feeder 
panels  of  the  main  switchboard,  but  they  may  be  smaller.  Also,  unless 
motor  generators  are  fixed  in  the  sub-station,  there  will  be  no  resistances 
or  rheostats,  but  where  they  are  employed,  as  suggested  in  previous 
chapters,  they  will  have  panels  of  their  own,  arranged  very  similarly 
to  the  panels  of  the  main  switchboard  designed  for  the  same  purpose. 

The  switchboards  for  such  sub-stations  as  the  pit  bottom  and 
distributing  points  in-bye  will  depend  for  their  size,  etc.,  upon  what 
they  have  to  deal  with.  Where  a  high  pressure  service  is  delivered 
at  the  pit  bottom,  such  as  3000  volts,  the  sub-station  switchboard  at 
the  pit  bottom,  should  be  ©f  something  the  same  character  as  the  cell 
arrangements  described  in  connection  with  the  main  switchboards. 
It  is  of  great  importance  that  attendants  at  the  pit  bottom  should  have 
very  little  chance  of  receiving  shocks.  One  arrangement  employed 


DISTRIBUTION    OF  POWER   BY   ELECTRICITY     255 


that  is  in  use  at  the  Powell  Duffryn  Co.'s  collieries,  supplied  by  the 
Westinghouse  Co.,  and  shown  in  Plate  13B,  where  3000  volts  are  taken 


M.  ro    r^3 


FIG.  109.— Diagram  of  Connections  of  a  Sub-station  Switchboard  at  a  Colliery, 
arranged  to  take  Current  from  a  Power  Service  at  6000  Volts  and  distribute 
it  at  the  Colliery  at  2000  Volts. 

to  the  pit  bottom  is,  brick  cells  are  built  in  a  chamber  near  the  pit 
bottom,  cut  out  of  the  coal  in  the  usual  way,  and  the  whole  of  the 


256 


ELECTRICITY  IN   MINING 


switch  gear,  circuit  breakers,  fuses,  etc.,  are  contained  in  these  cells, 
each  cell  being  separated  from  its  neighbour  by  a  substantial  brick  wall. 
Each  cell  is  closed  by  an  iron  door,  formed  by  a  single  casting  planed 
to  fit  the  doorway,  and  it  is  arranged  that  the  door  cannot  be  opened 


FIG.  110.— Back  of  Sub-station  Switchboard,  whose  Diagram  is  given  in  Fig.  109, 
showing  the  Transformers  and  Switch  Gear. 

until  the  connection  between  the  supply  cables  and  the  switch  gear 
is  broken,  this  being  accomplished  by  a  wheel  in  front  of  the  door, 
similar  to  that  used  for  rheostats  on  a  switchboard.  The  measuring 
instruments  for  each  of  the  cells  are  fixed  on  the  brickwork  above  the 
cells,  so  that  all  that  is  going  on  in  each  of  the  circuits  can  be  seen. 


PLATE  17 A. — Current  Transformer  for  use 
with  High  Tension  Switchboards,  as 
made  bv  Messrs.  Elliott  Bros. 


PLATE  17B. — Potential  Transformer  for 
Switchboards,  shown  in  Fig.  38. 


PLATE  17c.— Ferranti  Mining  Type, 
Oil  Immersed,  Gas-tight  Triple  Switch 
with  Fuses. 


PLATE  17D.— Messrs.  Reyrolle's  High  Tension, 
Three  Phase,  Oil  Enclosed  Switch,  with 
Oil  Tank  removed,  showing  Contacts. 


[To  face  p.  256. 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY    257 


As  explained,  in  every  sub-station  the  incoming  cable  from  the 
generating  station  takes  the  place  of  the  cable  from  the  generator 
itself,  and  this  rules  in  the  present  instance,  the  cables,  which  are 
three  phase  at  the  Powell  Duffiyn  Co.'s  works,  being  connected  to 
three  bus  bars  behind  the  cells,  the  connections  between  the  bus  bars 
and  the  switch  gear  in  the  cells  being  broken  when  the  door  is  open. 
One  of  the  cells  answers  to  the  generator  panel,  and  the  others  to  the 
feeder  panels.  For  continuous  current  two  wire  service  at  pit 


©    ©   ©        ©   ©   ©        ©   ©   © 

DC 


FIG.  111. — Diagram  of 
Double  -  pole  Distri- 
buting Board  made 
by  Messrs.  Berry, 
Skinner  &  Co. 


FIG.  112. — Diagram  of  Three-phase  Triple -pole 
Distributing  Board  made  by  Messrs.  Berry, 
Skinner  &  Co. 


bottoms,  etc.,  all  that  are  necessary  are,  switches  and  fuses  and  pre- 
ferably circuit  breakers  as  well,  to  disconnect  the  supply  cables  entirely 
from  the  seam,  just  as  the  generator  is  disconnected  from  the  switch- 
board, and  switches,  fuses,  and  circuit  breakers  for  each  set  of  cables 
leading  from  the  switchboard  to  any  given  district,  or  to  supply  any 
given  group  of  motors.  It  is  wise  also  to  have  ampere  meters  on 
both  supply  cables  from  the  service,  and  on  the  branch  feeder  cables, 
and  there  should  be  a  volt  meter  connected  to  the  supply  cables  from 
the  service.  The  whole  should  be  mounted  in  the  usual  way  upon 
slabs  of  marble  or  enamelled  slate  fixed  to  steel  framing,  and  should 

s 


258 


ELECTRICITY  IN   MINING 


either  be  placed  in  an  inaccessible  position,  as  in  the  deputy's  cabin, 
or  should  be  placed  in  a  locked  cupboard.  A  form  of  switchboard 
for  use  in  underground  engine  houses  that  is  finding  favour,  and  that 
is  employed  even  with  high  tensions,  consists  of  the  usual  panels 
fixed  to  iron  or  steel  framing,  with  the  switch  gear  instruments,  etc., 
mounted  upon  it,  the  whole  being  enclosed  within  a  substantial 
cupboard  of  wire  gauze  or  wire  netting.  The  arrangement  has  the 
advantage  that  the  apparatus  is  protected  from  accidental  contact,  the 
wire  cage  being  locked,  while  the  apparatus  itself  can  be  seen  from 
outside,  and  any  trouble  that  is  visible,  noted. 

For  distributing  points  in-bye,  where   the   space  is  often  very 


Copper  Core. 

Insulation. 

Outer  Copper. 

Lead  Sheathing. 

Iron  Armouring. 

Fuse  Box  Sockets. 


4- Way  Distributing  Fuse  Box 

(Cover  partly  removed,  showing  Section  of  Socket  and  Fuse.) 

FIG.  113. — Diagram  showing  Messrs.  Mavor  &  Coulson's  Fuse  Distributing  Box, 
used  with  their  Concentric  Cables. 

limited,  and  where  both  the  ground  and  any  supports  that  are  avail- 
able may  be  moved  frequently,  enclosed  iron  boxes,  made  sufficiently 
strong  to  withstand  the  rough  wear  and  tear  of  the  mine,  but  suffi- 
ciently portable  to  be  moved  fairly  easily  from  place  to  place,  are  to 
be  preferred.  The  iron  boxes  should  be  made  gastight,  and  the 
cables  entering  them  should  pass  through  gastight  glands.  There 
should  also  be  careful  provision  against  accidental  connection  between 
the  conductors  of  the  cables,  and  the  iron  of  the  boxes.  The  boxes 
should  always  contain  a  switch,  double,  triple  or  quadruple  pole,  for 
completely  disconnecting  the  supply  service  from  the  box,  and  from 


DISTRIBUTION   OF   POWER   BY  ELECTRICITY     259 

the  cables  supplied  from  the  box,  and  it  should  be  arranged  that  it  is 
not  possible  to  open  the  box  if  the  supply  switch  is  closed.  There 
are  several  arrangements  on  the  market  ensuring  this,  the  principle 
of  the  whole  of  them  being  the  same,  viz.  the  'door  as  it  closes  either 
pushes  in  a  plunger  which  unlocks  the  switch  gear,  or  turns  a  lever 
which,  when  the  door  is  open,  prevents  the  switch  contact  arm  from 
moving  into  contact.  Plates  15c,  16A  and  16n,  show  forms  of  these, 
also  Figs.  Ill  and  112.  Arrangements  may  be  made  for  connecting  the 
branch  cables  through  what  are  known  as  switch  fuses,  which  perform 
the  double  office  of  a  switch  and  a  fuse,  as  their  name  implies.  They 
consist  of  fuses  of  various  forms,  carried  sometimes  between  a  pair 
of  clips,  sometimes  in  other  ways,  but  there  is  always  a  handle  of 
insulating  material,  arranged  so  that  the  hand  of  the  operator  is 
protected  from  any  arc  or  spluttering  of  metal  that  may  take  place, 
when  either  the  fuse  "  blows  "  or  the  switch  is  opened.  If  any  excess 
current  passes  through  the  circuit  the  fuse  is  protecting,  the  fuse 
"  blows,"  and  if  it  is  desired  to  open  the  circuit,  it  may  be  done  by 
pulling  the  fuse  out  by  the  insulated  handle  provided  for  it.  It 
should  be  noted  that  it  should  be  part  of  the  arrangement  that  fuses 
can  only  be  changed,  and  the  fuse  switches  only  opened,  when  the 
supply  switch  is  also  open,  this  following  naturally  where  the 
boxes  cannot  be  opened  without  first  opening  the  supply  switch.  Fig. 
113  shows  one  of  Messrs.  Mavor  &  Coulson's  fuse  distributing  boxes, 
used  with  their  concentric  cables. 

For  gate  road  connecting  boxes,  such  as  must  be  employed  for 
coal-cutting  machines,  and  that  may  be  employed  in  certain  cases  for 
pumps,  and  other  apparatus  that  are  moving  forward,  all  that  is 
necessary  is,  provision  for  connecting  and  disconnecting  the  two, 
three,  or  four  cables,  and  for  their  being  automatically  broken  by 
fuses  blowing  in  case  of  short  circuits,  the  whole  being  enclosed  in  a 
portable  iron  box,  that  cannot  be  opened  when  the  circuit  is  closed. 
The  arrangement  of  the  connections  of  flexible  cables  for  coal-cutting 
machines  to  gate  end  boxes  will  be  discussed  in  connection  with  the 
machines  themselves. 


Measuring  Instruments  for  use  on  Main  and 
Sub=station  Switchboards 

Instruments  used  on  the  switchboards  are  of  the  moving  coil, 
moving  iron,  gravity,  and  hot  wire  types,  described  below.  Electro- 
static voltmeters  are  also  employed.  They  are  made  in  the  circular 
form,  with  dials  occupying  approximately  half  the  circumference,  in 
the  sector  form,  giving  a  large  dial  with  comparatively  small  move- 
ment, and  on  what  is  known  as  the  edgewise  pattern.  The  edgewise 


26o  ELECTRICITY   IN   MINING 

pattern  is  really  the  sector  form  with  the  circumference  of  the  sector 
arranged  for  the  scale,  and  turned  towards  the  attendant.  The 
circular  instruments  are  made  with  from  8 -inch  dials  up  to  11  inches, 
the  sector  instruments*  having  scales  rather  larger.  For  the  measure- 
ment of  continuous  current  strengths  above  a  certain  figure,  the  current 
is  shunted,  only  a  fraction  of  the  actual  current  measured  passing 
through  the  ampere  meters.  With  alternating  currents  transformers 
are  used,  where  the  currents  are  above  a  certain  strength,  the  current 
to  be  measured  passing  through  the  primary  coil,  the  secondary  coil 
of  the  transformer  being  connected  only  to  the  coil  of  the  instru- 
ment. Forms  of  transformers  are  shown  in  Plates  1?A  and  1?B,  for  use 
with  switchboard  instruments.  For  pressure  measurements,  resist- 
ances are  employed  with  continuous  currents,  and  transformers  with 
alternating  currents,  the  pressure  at  the  terminals  of  the  instrument 
being  a  small  fraction  of  that  of  the  actual  line  or  generator.  The 
sector  type  and  the  edgewise  type  are  frequently  arranged  with  their 
dials  illuminated  by  incandescent  lamps  fixed  above  the  switchboard, 
the  connections  being  brought  up  to  them  from  behind. 


Moving  Coil  Instruments 

In  the  moving  coil  instruments  there  is  a  permanent  magnet  with 
steel  pole  pieces,  enclosing  a  cylindrical  space,  in  which  a  wire  coil, 
carrying  a  pointer  at  its  centre,  is  pivoted.  It  is  all  important  that 
the  permanent  magnet  should  be  of  constant  coercive  force,  and  for 
this  purpose  a  special  alloy  of  steel,  in  which  tungsten  is  one  of  the 
components,  is  employed.  The  permanent  magnet  is  intended  to 
create  a  constant  magnetic  field  within  the  cylindrical  space  in  which 
the  coil  moves,  and  the  measure  of  the  strength  of  the  current  passing 
through  the  coil  is  the  angular  distance  through  which  it  is  moved 
away  from  the  zero  point.  The  angular  movement  of  the  pointer  is 
uniform  throughout  the  scale,  and  the  zero  point  may  be  either  on  the 
left  or  in  the  centre.  The  construction  of  the  instruments  of  the 
moving  coil  type  for  measuring  voltage,  and  for  measuring  current, 
is  exactly  the  same,  except  that  the  ampere  meters  are  provided  with 
shunts  consisting  of  a  number  of  strips  of  metal  whose  coefficient  of 
resistance  is  very  low.  They  are  virtually  millivolt  meters.  For 
instruments  required  to  measure  currents  up  to  100  amperes,  the 
shunt  is  usually  contained  within  the  instrument  itself,  but  for 
currents  above  that  figure  it  is  carried  in  a  separate  case. 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     261 


Gravity  Instruments 

In  these  instruments  there  is  a  circular  coil  of  wire,  and  inside 
the  coil  a  crescent-shaped  piece  of  iron  attached,  by  means  of  a  radial 
member,  to  a  pivot  upon  which  the  pointer  works.  The  instrument 
is  fixed  vertically,  and  in  that  position  the  crescent  of  iron  falls  to 
its  lowest  point  when  no  current  is  passing.  When  a  current  passes 
through  the  coils  of  the  instrument,  the  crescent  is  moved  out  of  its 
position  by  the  magnetic  field  created  within  the  cylindrical  space 
inside  the  coil,  the  needle  pointer  moving  over  the  dial  in  unison 
with  it.  The  indications  of  this  instrument  are  not  uniform.  The 
instrument  is  made  to  measure  currents  up  to  certain  figures,  as  from 
0  to  120  volts,  from  0  to  100  amperes,  and  so  on.  In  these  cases  the 
indications  up  to  70  in  voltmeters  are  very  small,  the  scale  then 
gradually  spreading  out  so  that  when  the  instrument  is  reading  what 
is  usually  the  normal  pressure,  100  or  110,  differences  of  even  1  volt 
are  easily  distinguished.  On  the  ampere  meters  the  readings  com- 
mence for  100  volt  ampere  meter  at  from  10  to  13  amperes,  but  the 
spaces  of  the  scale  are  very  small,  until  the  neighbourhood  of  the 
figures  the  instrument  is  intended  to  read  normally  are  reached, 
after  which  the  spaces  become  smaller  again  for  high  tension. 

Hot  Wire  Instruments 

These  instruments  depend  upon  the  fact  that  when  a  current 
passes  through  a  wire  and  heat  is  liberated,  the  heat  causes  expan- 
sion of  the  wire,  the  expansion  being  measured  by  the  motion  of  a 
pointer  over  a  scale. 

There  are  two  forms  of  the  instrument.  In  one  the  wire  which  is 
to  be  heated  is  enclosed  in  a  long  tube,  with  a  cylindrical  box  at  its 
end,  carrying  the  pointer  and  dial  on  its  face.  The  tube  may  be 
fixed  either  in  a  vertical  or  in  a  horizontal  position,  the  dial  being 
arranged  accordingly.  In  the  other  form  of  apparatus  the  wire  is 
enclosed  within  a  cylindrical  case,  fixed  vertically,  with  the  dial 
occupying  the  upper  portion.  In  both  forms  of  instrument  very  thin 
platinum  silver  wire  of  high  resistance  is  employed,  the  wire  used  in 
the  tube  form  being  very  long,  while  in  the  dial  form  is  short.  In 
both  cases  the  wire  is  stretched  tight,  it  being  carried  over  pulleys  at 
the  top  and  bottom  of  the  tube  in  the  long  form,  and  is  kept  in 
tension  by  means  of  a  spiral  spring.  In  the  tube  form,  the  tube 
itself  is  made  in  two  halves  of  two  different  metals,  arranged  so  that 
the  expansion  of  the  two  from  the  heat  generated  in  the  wire  shall 
neutralize  each  other,  and  the  indications  on  the  dial  be  entirely 
confined  to  the  elongation  of  the  wire  from  the  heat  of  the  current. 


262  ELECTRICITY  IN   MINING 

In  the  hot  wire  ampere  meters,  the  shunt  principle  is  also  applied  in 
a  similar  manner  to  that  of  the  moving  coil  apparatus.  The  motion 
of  the  needle  of  the  circular  and  edgewise  form  of  the  instrument  is 
damped  by  means  of  a  permanent  magnet,  between  the  poles  of 
which  a  thin  aluminium  disc  works,  so  that  the  needle  comes  quickly 
to  rest. 

The  Electrostatic  Voltmeter 

The  electrostatic  voltmeter  is  employed  for  measuring  high 
pressures.  The  apparatus  works  by  reason  of  the  attraction  between 
oppositely  charged  conductors  at  different  electrical  pressures, 
and  the  repulsion  of  similarly  charged  bodies,  the  attraction  and 
repulsion  being  in  proportion  to  the  square  of  the  difference  of  the 
pressure.  There  are  two  principal  forms  made,  known  respectively 
as  the  multicellular  and  the  vane  instruments.  In  the  multi- 
cellular  instrument,  which  is  intended  for  measuring  comparatively 
low  voltages,  there  are  a  number  of  small  insulated  cells,  formed 
of  triangular  brass  plates,  fixed  into  slots  cut  in  a  vertical  back 
piece,  the  spaces  between  the  plates  forming  the  cells.  Two  sets 
of  cells  are  fixed  with  their  plates  horizontal,  and  opposite  each 
other,  on  a  vulcanite  support.  The  mo  via  g  member  of  the  system 
consists  of  a  number  of  vanes  fixed  horizontally  upon  a  light  vertical 
spindle,  in  such  a  position  that  one  side  of  each  vane  lies  in  each 
of  two  cells  opposite  each  other.  The  vanes,  with  their  spindle, 
are  suspended  by  a  fine,  iridio-platinum  wire  from  a  torsion  head  at 
the  top  of  a  vertical  brass  tube  surmounting  the  instrument.  The 
pressure  is  communicated  to  the  vanes  through  the  wire  suspension. 
When  a  pressure  is  delivered  to  the  cells  on  the  one  hand,  and  to  the 
vanes  on  the  other,  the  vanes  turn  in  a  horizontal  plane,  the  sus- 
pending wire  carrying  a  light  aluminium  pointer  over  a  horizontal 
circular  scale  fixed  at  the  top  of  the  instrument. 

Switches,  Puses,  and  Circuit  Breakers 

It  was  explained  in  describing  switchboards,  that  switches  were 
provided  for  connecting  and  disconnecting  generators,  feeders,  etc. 
Modern  switches  are  all  constructed  on  certain  main  lines.  They 
must  all  conform  to  certain  conditions.  In  every  switch  there  is 
a  moving  contact  bar,  which  makes  connection  between  two  fixed 
contact  blocks,  or  springs,  to  which  the  ends  of  the  cable  connected 
to  the  circuit  to  be  controlled  by  the  switch  are  brought,  and  the 
contact  bar  must  in  all  cases  be  of  sufficient  size  to  allow  the  passage 
of  the  largest  current  the  switch  will  have  to  control,  without  an 
appreciable  rise  of  temperature.  The  surfaces  of  contact  between 


DISTRIBUTION   OF  POWER  BY  ELECTRICITY    263 

the  contact  bar  and  the  fixed  contacts  must  also  be  of  sufficient  cross- 
section  to  allow  the  passage  of  the  current  from  one  contact  to  the 
contact  bar,  and  from  the  contact  bar  to  the  other  contact,  again 
without  appreciable  rise  of  temperature.  The  rule  adopted  by  the 
principal  makers  of  switches  is  1000  amperes  per  square  inch  for  the 
contact  bar,  and  75  amperes  per  square  inch  for  the  contact  services, 
with  the  maximum  currents  the  switches  are  designed  for.  Switches 
must  also  be  so  arranged  that  on  opening  a  circuit  no  arc  can  form 
between  either  the  fixed  contact  pieces,  or  between  them  and  the 
moving  contact  bar.  As  explained  before,  when  a  circuit  is  opened, 
especially  circuits  in  which  there  are  a  number  of  coils  of  wire  in 
which  a  current  is  passing,  the  return  to  the  circuit  of  the  energy 
delivered  to  the  magnetic  field  when  the  circuit  was  closed,  and  that 
delivered  to  the  electrostatic  condenser,  create  a  very  large  increase 
of  pressure,  which  causes  a  spark  to  pass  across  the  break  at  the 
moment  the  circuit  is  opened ;  and  if  the  break  is  not  made  of  such 
a  size  that  the  spark  cannot  persist,  in  a  very  short  interval  of 
time  an  arc  will  be  formed,  just  as  in  the  arc  lamp,  and  the  working 
portions  of  the  switch,  the  stationary  contacts,  and  sometimes  the 
moving  contact  bar,  will  be  seriously  damaged,  the  arc  producing 
temperatures  that  quickly  melt  brass,  copper,  etc.,  and  that  destroy 
the  insulating  material  upon  which  the  switch  is  fixed. 

There  are  two  principal  lines  upon  which  switches  are  con- 
structed, which  are  really  variations  of  one  type.  A  favourite  form 
is  the  knife  switch,  in  which  a  knife  blade,  constructed  of  copper,  or, 
in  the  case  of  small  switches,  of  brass,  is  the  moving  contact  bar,  and 
is  forced  between  two  spring  stationary  contact  pieces,  when  the 
circuit  is  closed.  This  form  of  switch  is  made  either  "  slow  break  " 
or  "  quick  break."  Slow  break  knife  switches  are  made  for  currents 
with  pressures  below  300  volts,  and  in  them  the  quickness  of  the 
hand  is  depended  upon  to  break  the  arc.  In  quick  break  knife 
switches  a  spring  comes  into  operation  at  the  instant  the  moving 
contact  bar  is  leaving  the  fixed  contacts,  takes  charge  of  the  moving 
contact  bar,  and  throws  it  quickly  back  out  of  sparking  distance,  the 
moving  contact  bar  being  constructed  so  as  to  move  freely,  apart 
from  the  insulated  handle,  or,  as  it  is  termed,  with  a  loose  handle. 
The  Westinghouse  Co.,  the  Ferranti  Co.,  and  others  have  developed 
standard  knife  switches,  in  which  the  knife  blade  is  always  of  one 
size  and  one  thickness,  and  a  number  of  knife  blades  are  put  together 
in  one  switch  for  different  strengths  of  current.  Thus  a  single  knife 
blade  will  carry  currents  up  to  a  certain  number  of  amperes,  two 
knife  blades  up  to  double  the  number  of  amperes,  and  so  on. 

Switches  are  made  to  open  the  circuit  of  one  cable,  of  two,  three, 
or  four  cables,  as  required,  and  are  termed  single  pole  when  they 
break  only  one  cable,  double  pole  for  two,  triple  pole  for  three,  and 


264  ELECTRICITY  IN   MINING 

so  on.  Single-pole  switches  are  employed  for  opening  the  circuit 
of  one  cable  of  a  continuous  current  system,  double-pole  switches 
for  opening  both  cables  of  a  continuous  current  system,  triple-pole 
for  opening  the  three  cables  of  either  a  three-wire  or  three-phase 
system,  and  quadruple-pole  for  the  four  wires  of  a  two-phase  system. 
It  should,  perhaps,  be  mentioned  that  it  is  necessary  to  open  all 
three  cables  of  a  three-phase,  and  all  four  cables  of  a  two-phase 
system  simultaneously,  except  in  those  cases  where  lamps  or  other 
apparatus  are  connected  between  the  different  phases.  Fig.  114  shows 
a  form  of  double-pole  switch,  enclosed  in  an  iron  case,  intended  for 
mining  work. 

Switches  are  also  distinguished  as  single  and  double  break,  ac- 
cording to  whether  the  contact  bar  makes  two  breaks  between  itself 


FIG.  114. — Double-pole  Switch  enclosed  in  an  Iron  Case  made  by  the 
Electric  &  Ordnance  Co. 

and  the  stationary  contact  points,  or  whether  there  is  only  one  break 
between  the  contact  bar  and  the  stationary  contact  itself.  Switches 
are  very  rarely  made,  and  should  not  be  used  where  it  can  be 
avoided,  in  which  the  break  is  between  the  contact  bar  and  the 
stationary  contacts,  because  in  that  case  the  current  has  to  pass 
through  a  hinge  or  some  similar  arrangement  to  reach  the  contact 
bar,  and  this  is  often  a  source  of  trouble. 

The  knife  switch  has  been  developed  for  single,  double,  triple, 
and  quadruple  switches,  as  it  lends  itself  very  readily  to  construction 
such  that  two,  three,  or  four  knife  blades  can  be  forced  between  their 
respective  contacts  by  a  single  insulated  handle,  and  can  be  with- 
drawn simultaneously  by  the  same  means.  They  have  also  been 
developed  for  single  and  double  throw  switches.  The  single-throw 
switch  is  the  one  that  has  been  described,  by  which  a  circuit  is 


PLATE  ISA. 


-Porcelain  Handle  Fuses,  made  by 
Messrs.  Revrolle. 


PLATE    1SB. — Ferranti    Carbon 
Break  Circuit  Breaker,  closed. 


PLATE  18c. — Ferranti  Carbon  Break  Circuit 
Breaker,  open. 


PLATE  1 80. —Ferranti  Triple  Hole 
Overload,  and  no  Voltage  Circuit 
Breaker,  with  Cover,  not  Gas- 
tight. 

[To  face  p.  264. 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     265 

closed  or  opened.  The  double-throw  switch  is  the  one  described  in 
connection  with  the  "  independent "  distribution  system,  by  which  a 
current  is  thrown  over  from  one  circuit  to  another.  In  America 
they  are  called  throw-over  switches,  because  the  current  is  thrown 
over  from  one  generator  to  another,  or  from  one  feeder  to  another. 
In  the  double-throw  knife  switch,  which  again  is  made  for  single, 
double,  triple,  and  quadruple  pole,  there  are  one,  two,  three,  or  four 
pairs  of  knife  blades  fixed  to  one  handle,  each  pair  being  at  such  an 
angle  with  each  other  that  when  one  set  of  knives  is  in  connection 
with  its  contacts,  the  other  set  is  well  clear  of  the  other  contacts. 
There  are  one,  two,  three,  or  four  sets  of  contacts  facing  each  set  of 
knife  blades,  and  the  operation  of  throwing  over  consists  in  quickly 
forcing  one  set  of  blades  between  its  contacts,  at  the  same  instant 
the  other  set  of  blades  leaving  its  contacts.  In  another  form  of 
double-throw  knife  switch,  there  is  only  one  set  of  blades  which 
stand  vertical  between  two  sets  of  contacts  fixed  on  a  horizontal 
base,  and  the  one  set  of  blades  is  thrown  quickly  from  one  set  of 
contacts  across  through  an  arc  of  180°  to  the  other  set.  Contact  is 
made  with  one  set  of  contacts  by  one  edge  of  the  blades,  and  with 
the  other  set  of  contacts  by  the  other  edge  of  the  blades. 

In  the  other  principal  form  of  switch,  which  is  sometimes  known 
as  the  chopper  switch,  and  sometimes  by  other  names,  the  fixed 
contacts  are  farther  apart,  and  the  contact  bar  is  made  with  a  straight 
piece  ending  in  a  bridge  of  some  form,  arranged  to  fill  up  the  space 
between  the  two  fixed  contact  pieces.  In  the  form  that  was  a 
favourite  some  years  ago,  the  moving  contact  bar  was  built  up  of  a 
number  of  thin  sheets  of  copper,  bent  round  a  bar  at  the  centre,  to 
which  the  handle  or  moving  mechanism  was  connected,  and  the 
ends  being  spread  out  in  the  form  of  a  brush,  the  brush  sweeping 
down  between  the  fixed  contact  pieces.  The  chopper  switch  and  the 
brush  switch  do  not  lend  themselves  so  readily  to  double-pole,  triple- 
and  quadruple-pole  construction  as  the  knife-switch  construction 
does.  The  action  of  the  hand  in  closing  or  opening  the  switch  has 
to  be  transmitted  to  the  moving  contact  bars,  through  bars  of  insu- 
lating material,  and  it  is  sometimes  difficult  to  fit  these  so  that  the 
whole  of  the  contacts  go  in  and  out  of  contact  simultaneously,  and  as 
it  is  necessary  to  provide  pins  working  in  holes,  or  something  similar, 
in  the  bars  of  insulating  material,  these  also  wear  with  time,  and 
tend  to  throw  the  switch  out  of  gear. 

For  large  currents  a  form  of  switch  has  been  developed  in  which 
the  contact  is  made  between  substantial  copper  surfaces,  while  the 
circuit  is  closed,  but  in  which  the  final  break  is  made  between 
carbon  contacts  fixed  for  the  purpose,  and  arranged  to  be  renewed 
when  burned  out.  The  masses  of  copper  may  be  in  the  knife  form,  or 
may  consist  of  laminated  copper  bearing  against  or  between  stationary 


266  ELECTRICITY  IN   MINING 

contact  pieces ;  but  in  all  cases  there  is  an  auxiliary  lever  carrying 
the  auxiliary  carbon  contact  attached  to  the  contact  bar,  and  there  is 
an  auxiliary  fitting  on  the  switch  base,  carrying  the  auxiliary  fixed 
carbon  contact  piece,  which  is  arranged  to  engage  with  the  carbon 
contact  piece  on  the  moving  contact  bar.  When  the  switch  is  closed, 
the  carbon  contacts  come  into  connection  first,  the  handle  operating 
the  switch  being  arranged  in  this  manner,  the  copper  contact  pieces 
then  coming  up  as  the  switch  is  forced  home,  and  being  driven  into 
their  places.  When  the  switch  is  opened,  the  handle  first  looses  and 
releases  the  copper  moving  contact  piece,  throws  it  clear  of  the 
stationary  contact  piece,  and  then,  as  the  handle  moves  on,  it  throws 
back  the  moving  carbon  contact  piece,  any  spark  that  passes  or  arc 
that  is  formed  being  between  the  carbon  contacts.  This  form  of 
switch  is  arranged  for  double  and  triple  pole,  the  different  switches 
for  the  different  poles  being  fixed  one  under  the  other,  a  vertical  rod 
operating  the  three,  the  rod  being  actuated  by  a  single  handle. 


Fuses 

Fuses  are  intended  to  protect  both  the  coils  of  generators,  motors, 
and  cables  from  the  passage  of  currents  that  will  heat  them  to  a 
dangerous  extent,  and  will  damage  the  insulation.  They  are  all 
constructed  on  the  principle  that  certain  metals  have  a  lower  melting 
point  than  copper,  and  that  all  metals  in  a  very  small  section  will  be 
melted  or  disintegrated  when  a  current  passes  through  them  of  a 
certain  strength.  The  principal  metals  employed  for  fuses  are  lead, 
tin,  and  copper.  Alloys  of  tin  and  lead  a*nd  other  metals  are  also 
employed.  Aluminium  is  utterly  unsuitable  for  fuses,  because  when 
it  is  heated  by  the  passage  of  a  current  through  it,  an  oxide  is  formed 
on  the  outside  of  the  wire,  which  has  a  considerable  factor  of  cohesion, 
and  holds  the  wire  itself  together  for  some  time  after  the  substance 
has  really  been  melted,  and  therefore  does  not  open  the  circuit. 
Lead  and  tin,  and  the  alloys  of  lead,  with  tin  and  other  metals,  have 
two  distinct  disadvantages,  they  oxidize  very  freely  when  the  current 
is  passing,  and  from  the  moment  they  are  put  into  service  the  current 
they  will  stand  without  fusing  decreases,  so  that  unless  they  are 
renewed  somewhat  frequently  they  are  apt  to  break  circuit  at  very 
awkward  times,  such  as  when  an  additional  load  is  thrown  on  a 
distributing  cable,  a  load  that  carries  no  dangerous  heating  properties, 
and  that  the  cable  should  carry  very  conveniently.  The  other  objec- 
tion to  lead,  tin,  and  their  alloys  is  when  the  fuse  "  blows,"  the 
molten  metal  is  scattered  all  round  the  place  where  the  fuse  is,  and 
there  is  usually  a  good  deal  of  damage  done  to  the  enamelled  slate,  or 
whatever  the  fuse  may  be  mounted  on.  For  these  reasons  copper  has 


DISTRIBUTION   OF   POWER   BY  ELECTRICITY    267 

been  employed  a  good  deal,  and  a  thin  copper  wire  properly  pro- 
portioned makes  a  very  good  fuse  indeed.  It  does  not  oxidize  as 
freely  as  the  tin  and  lead  fuses  do.  It  does  not  splutter  so  much  as 
they  do,  and  it  is  generally  more  reliable.  The  fuse,  whatever  its 
form,  consists  of  a  short  length  of  wire  that  will  carry  the  current 
the  cable  or  generator  is  to  deal  with  normally,  but  that  will  melt, 
owing  to  the  heat  generated  in  it,  if  a  current  of  50  per  cent.,  or  100 
per  cent.,  or  whatever  the  fuse  may  be  set  to  "  blow "  at,  arrives. 
There  are,  however,  some  difficulties  in  connection  with  fuses.  It  is 
necessary  in  order  that  the  fuse  wire  may  be  included  in  the  circuit, 
that  it  shall  be  connected  to  metal  blocks,  to  which  the  ends  of  the 
wires  of  the  circuit  shall  also  be  connected,  and  these  blocks  absorb  a 
certain  portion  of  the  heat  liberated  in  the  fuse  wire,  and  they  also 
dissipate  a  portion  of  the  heat.  This  results  in  fuses  varying  in  their 
action  with  the  temperature  of  the  surrounding  atmosphere.  In  a 
cold,  draughty  passage  a  fuse  will  often  allow  a  very  dangerous 
current  to  pass  where  it  would  "  blow  "  in  a  warmer  atmosphere,  such 
as  an  engine-room,  with  a  very  much  smaller  and  sometimes  less 
than  the  normal  current.  This  has  led  to  the  development  of  what 
are  termed  "  enclosed "  fuses,  consisting  of  wires  of  various  metals 
enclosed  inside  of  glass  or  metal  tubes,  the  tubes  being  filled  with 
various  substances,  such  as  oil  with  a  high  flash  point,  sand,  asbestos, 
and  chemicals  which  are  designed  to  extinguish  the  arc  which  is 
formed  when  the  fuse  blows.  It  will  be  understood  that  one  of  the 
troubles  in  connection  with  fuses  is  the  possibility  of  an  arc  being 
formed  between  the  severed  ends  of  the  fuse  wire.  Enclosed  fuses 
are  gradually  coming  into  service,  though  there  have  been  some 
complaints,  in  the  case  of  fuse  wires  enclosed  in  oil,  that  the  oil  has 
been  fired  by  the  arc  formed,  and  the  enclosing  vessel  has  exploded. 
A  variation  of  the  enclosed  fuse  which  is  made  by  Messrs.  John 
Fowler  &  Co.,  of  Leeds,  is  the  asbestos-covered  fuse.  In  this  fuse, 
wire  of  a  certain  section  is  covered  with  asbestos  to  a  standard  thick- 
ness, that  it  has  been  calculated  will  prevent  the  formation  of  any 
arc,  and  that  will  prevent  the  escape  of  heat  from  the  fuse.  The 
standard  fuse  is  made  for  currents  of  20  amperes,  and  a  circuit  is 
fused  for  any  current  by  simply  multiplying  the  number  of  single 
fuse  wires  fixed  between  fuse  blocks.  Another  point  in  connection 
with  fuses  is  the  matter  of  the  replacement  of  the  fuse  wires  after  a 
fuse  has  "  blown."  Time  is  often  of  considerable  importance,  and  the 
fuse  blocks  are  also  nearly  always  in  such  positions  that  a  man  will 
receive  a  shock  if  the  circuit  to  which  the  fuse  is  to  be  connected  is 
alive,  in  the  process  of  replacing  the  fuse  wire.  Hence  a  line  of 
fuses  has  been  worked  out,  in  which  the  fuse  wire  is  stretched 
between  pieces  of  metal,  held  sometimes  by  bridges  of  vulcanized 
fibre,  sometimes  by  bridges  of  porcelain  ;  and  again  the  bridge  may 


268  ELECTRICITY   IN   MINING 

be  a  glass  or  porcelain  tube,  the  porcelain  tube,  as  explained  above, 
in  some  cases  forming  the  handle  by  which  the  fuse  is  replaced. 
In  any  case,  the  bridge  is  employed  to  handle  the  fuse  by,  and  the 


FIG.  115. — Fuses  for  Mining  Work  enclosed  in  Iron  Case.    The  Fuse  on  the 
Left  has  a  Porcelain  Handle  for  removing  it  and  for  using  it  as  a  Switch. 

metal  terminals  to  which  the  fuse  wire  is  attached  are  arranged  to  be 
pushed  between  metal  springs,  connected  to  metal  blocks  forming 
part  of  the  circuit  to  be  protected.  Examples  of  these  are  shown  in 
Plate  ISA  and  Fig.  115. 


Circuit  Breakers 

The  circuit  breaker  is  intended  to  perform  the  same  office  as  the 
fusible  cut-out,  but  to  be  more  certain.  In  all  forms  of  circuit 
breaker  an  electro-magnet  is  employed  to  open  the  circuit,  and  it 
does  so  by  either  releasing  a  trip  action,  or  by  moving  a  lever  bodily. 
In  all  forms  there  is  a  provision  for  tripping  by  hand  without  danger 
to  the  attendant.  Circuit  breakers  are  made  for  breaking  the  circuit 
of  a  single  cable  with  continuous  current,  when  an  overload  arrives, 


DISTRIBUTION   OF   POWER   BY   ELECTRICITY     269 

or  when  the  pressure  of  the  service  falls  below  a  certain  figure,  and 
also  to  open  the  circuit  if  a  reverse  current  arrives,  as  when  machines 
are  connected  in  parallel  and  one  of  them  is  motoring.  They  are  con- 
structed to  break  the  circuit  of  both  cables  of  a  two- wire  service,  of 
the  three  cables  of  a  three-wire  service,  and  the  two,  three,  or  four 
cables  of  an  alternating  current  service.  Plates  13c,  16c  and  16D,  and 
18s,  18c,  and  18D,  show  forms  of  Ferranti  circuit  breakers.  There 
are  also  special  arrangements  described  below,  for  delaying  the  opera- 
tion of  the  circuit  breaker. 


Time  Limit  Circuit  Breakers 

For  a  power  service  the  fuse  has  one  advantage  over  the  circuit 
breaker,  it  is  not  usually  so  quick  in  action,  and  the  quickness 
of  action  of  the  circuit  breaker  in  some  cases  leads  to  inconvenience. 
In  the  time  limit  circuit  breaker  a  certain  time  must  elapse,  after  the 
current  arrives  at  the  circuit  breaker,  before  it  operates.  Time  limit 
circuit  breakers  are  principally  of  two  forms,  those  in  which  clockwork 
is  employed,  and  those  in  which  electro-magnetic  induction  is  made  use 
of  to  interpose  a  lagging  action  upon  the  apparatus.  In  the  apparatus 
in  which  clockwork  is  employed,  a  clock  movement  is  held  from 
running  down  by  a  pawl,  which  falls  into  a  recess  in  a  wheel  con- 
trolling the  train.  On  another  wheel  driven  by  the  train  is  a  contact, 
which,  when  the  clock  is  released,  moves  into  contact  with  a  fixed 
contact  provided  for  the  purpose,  and  in  so  doing  completes  the 
circuit  in  which  the  coils  of  an  electro  magnet  operating  the  trip 
action  of  the  circuit  breaker  are  included. 

Another  form  of  time  limit  circuit  breakers  are  operated  by  air 
pressure.  In  these  apparatus  the  core  of  a  solenoid  is  lifted  or 
depressed  in  opposition  to  air  or  oil  pressure,  so  arranged  that  it 
requires  a  definite  time  to  overcome  it,  and  if  the  short  circuit  is 
removed  before  this  period  has  elapsed,  the  circuit  breaker  is  not 
opened. 


The  Ferranti  Alternating  Current  Time  Limit 
Relay  for  Circuit  Breakers 

In  this  apparatus  there  is  an  electro-magnet  with  a  core  of 
laminated  iron  plates,  its  coils  being  energized  by  the  secondary 
current  of  a  transformer,  the  primary  of  which  is  included  in  the 
circuit  to  be  controlled.  The  poles  of  the  magnet  are  provided 
with  what  Mr.  Ferranti  has  called  "shading  coils,"  that  is  to 
say,  coils  placed  on  the  pole  pieces,  each  coil  closed  on  itself, 


270  ELECTRICITY  IN   MINING 

and   the   coils   becoming  smaller    as  the  core  of  the   pole    piece 
becomes  smaller. 

A  copper  disc  is  pivoted  between  the  poles  of  the  electro-magnet, 
and  is  free  to  revolve,  its  motion  being  controlled  by  a  hanging 
weight,  and  retarded  by  an  adjustable  damping  permanent  magnet 
covering  a  portion  of  its  edge.  The  hanging  weight  carries  a  pivoted 
lever  with  a  contact  which,  when  the  apparatus  operates,  completes 
the  circuit  of  the  tripping  coils  of  the  circuit  breaker.  When  the 
feeder  that  is  under  the  control  of  the  apparatus  has  an  excessive 
current  passing  through  it,  the  copper  disc  is  set  in  motion,  and,  in 
revolving,  winds  up  the  cord  to  which  the  hanging  weight  is  attached. 
When  the  weight  has  been  drawn  up  a  certain  definite  distance,  that 
is,  when  the  copper  disc  has  made  a  certain  number  of  revolutions, 
the  moving  contact  referred  to  makes  connection  with  the  fixed 
contact,  closes  the  circuit  of  the  tripping  coil,  and  opens  the  circuit 
breaker.  A  certain  definite  period,  ranging  up  to  thirty  seconds,  must 
elapse,  during  which  the  copper  disc  is  revolving,  and  gradually 
bringing  the  moving  contact  piece  towards  the  fixed  contact  piece, 
before  the  circuit  breaker  can  operate.  If,  before  this  period  has 
elapsed,  the  overload  is  removed,  the  copper  disc  ceases  to  revolve, 
the  weight  revolves  it  in  the  opposite  direction  by  itself,  running 
down,  the  moving  piece  being  at  the  same  time  carried  away  from 
the  fixed  contact  piece.  The  relay  can  be  set  to  operate  with  any  per- 
centage of  overload  that  may  be  desired.  It  is  usually  set  for  a  25 
per  cent,  overload,  but  it  can  be  set  for  50  or  more,  as  required.  In 
the  working  of  the  apparatus,  the  time  which  elapses  before  the 
circuit  breaker  opens  the  circuit  is  inversely  proportional  to  the 
degree  of  overload.  Thus,  with  a  slight  overload  the  copper  disc  will 
revolve  slowly,  and  the  moving  contact  will  take  a  comparatively 
long  time  in  completing  the  trip  circuit.  With  a  very  bad  short 
circuit  the  copper  disc  will  revolve  very  quickly,  the  trip  circuit  will 
be  closed  in  a  very  short  interval  of  time,  and  the  circuit  will  be  very 
quickly  opened. 


Atkinson's  Time  Limit  Circuit  Breaker 

In  this  apparatus  there  is  a  tube  filled  with  oil,  in  which  a  metal 
ball  is  free  to  roll  from  end  to  end.  At  the  end  of  the  tube  is  a 
small  chamber  containing  two  contacts,  one  fixed  and  the  other 
movable;  the  movable  contact  having  a  mica  vane  attached  for 
the  purpose  of  damping  its  motion.  These  two  contacts  close  the  trip 
circuit.  Under  normal  conditions  the  tube  is  inclined  at  an  angle 
slightly  out  of  the  horizontal,  so  that  the  ball  remains  at  the  end  of 
the  tube  away  from  the  contact  chamber.  When  the  overload 


DISTRIBUTION   OF  POWER  BY   ELECTRICITY    271 

arrives,  the  tube  is  slightly  inclined  towards  the  contact  chamber, 
the  ball  then  rolls  to  the  contact  chamber,  and  forces  the  moving 
contact  into  connection  with  the  fixed  one.  The  time  taken  can  be 
regulated  between  three  minutes  and  sixty  minutes,  and  the  current 
between  60  and  100  per  cent,  overload.  In  the  event  of  a  short 
circuit,  the  tube  is  inclined  so  much  that  the  weight  of  the  moving 
contact  is  sufficient  to  bring  it  into  connection  with  the  fixed  contact, 
the  relays  operating  at  once. 


CHAPTEE   VI 

THE    APPLICATION    OF    ELECTRICITY    TO 
DRIVING  MACHINES,  ETC.,  IN  MINES 

The  Electric  Motor 

THEEE  are  practically  only  two  forms  of  electric  motor  in  use  at  the 
present  time  suitable  for  mining  work,  the  continuous  current  and 
the  three-phase  motors.  The  two-phase  motor  is  also  used  occasion- 
ally, but  it  is  practically  the  same  as  the  three-phase,  it  having  two 
sets  of  coils  on  the  stator,  with  four  terminals  and  four  slip  rings  on 
one  rotor  in  place  of  three.  The  single-phase  motor,  though  it  is 
being  steadily  developed,  has  not  yet  reached  the  point  at  which  it  is 
suitable  for  mining  work.  The  continuous-current  motor  is  the  con- 
tinuous-current generator,  having  current  delivered  to  it  from  a  supply 
of  electricity,  in  place  of  being  driven  by  mechanical  power.  Plate 
19A  shows  the  parts  of  a  modern  continuous-current  motor.  It  should 
be  noted  that  the  mechanical  power  any  electric  motor  will  furnish 
is  approximately  20  per  cent,  less  than  the  power  required  to  drive 
it  as  a  generator,  the  machine  doing  full  work  in  each  case.  The 
reason  is,  the  work  any  dynamo  will  perform,  whether  as  generator 
or  motor,  is  limited  by  the  current  its  wires  will  accommodate  with- 
out unduly  heating,  and  the  pressure  it  will  generate  when  going  at 
a  safe  speed.  When  the  dynamo  is  run  as  a  generator,  the  charge  for 
conversion  has  to  be  added  to  the  output,  this  making  the  total  power 
required  to  be  delivered  to  the  driving  axle.  When  it  is  run  as  a 
motor,  the  charge  for  conversion  is  subtracted  from  the  total  electrical 
power  delivered  to  its  terminals.  The  continuous-current  motor  is 
wound  just  as  a  generator  is,  as  series,  shunt,  or  compound,  and  each 
form  has  its  own  properties. 

The  series- wound  motor  develops  a  powerful  torque  on  starting, 
more  powerful  than  either  of  the  other  forms,  but  its  speed  varies 
very  considerably  with  changes  of  load,  and  it  is  therefore  not  so 
suitable  for  work  where  uniform  speed  is  required  or  is  advantageous. 
The  reason  is,  the  power  a  motor  will  develop  depends  directly  upon 

272 


DRIVING   MACHINES   BY   ELECTRICITY 


273 


the  strength  of  the  magnetic  field,  and  upon  the  strength  of  the 
current  passing  through  its  armature  coils.  When  a  continuous- 
current  series-wound  motor  is  first  started,  the  back  pressure  being 
small,  a  powerful  current  passes  through  both  armature  and  field 


MMMAA/V 

FIELD  COILS- 


SUPPLV  CABLES 


STARTING  SWITCH 
sr  RESISTANCE 


FIG.  116. — Diagram  of  Connections  for  starting  a  Series-wound  Motor. 

coils,  with  the  result  that  the  field  is  very  strong,  and  the  motor 
develops  a  powerful  torque.  On  the  other  hand,  with  changes  of 
load,  when  the  machine  is  running  normally,  a  decrease  of  load 
allowing  it  to  increase  its  speed  reduces  both  the  current  in  the 


SUPPLY   CABLES 


Fieuo  Goms 


FIG.  117.— Diagram  of  Connections  for  starting  a  Shunt-wound  Motor. 

armature  and  the  strength  in  the  field  coils,  this  latter  causing  a 
further  increase  of  speed,  and  so  on. 

The  shunt-wound  motor  gives  a  very  nearly  uniform  speed  with 
varying  load.     In  fact,  it  is  possible  to  construct  a  shunt-wound 

T 


274 


ELECTRICITY   IN   MINING 


rO  90-1    p-O 


FIG.  118.— Diagram  of  Connections  of  Johnson  &  Phillips'  Motor  Starting  Box,  with 
Double-pole  Switch,  Fuses,  and  Starting  Resistance  for  Continuous  Currents. 


DRIVING    MACHINES   BY   ELECTRICITY         275 

motor  whose   variations   of  speed  within,  a  certain  range  shall  be 
negligible.      The  shunt-wound  motor  is   also   self-governing.     The 
electrical  energy  supplied  to  an  electric  motor  may  be  divided  into 
two  portions,  that  which  is  converted  into  heat  in  the  coils  of  the 
armature,  and  which  is  employed  in  creating  the  magnetic  field  and 
in  overcoming  the  frictional  resistance,  and  that  which  does  useful 
work.     Every  electric  motor,  when  running,  generates  a  back  pres- 
sure, in  opposition  to  the  pressure  of  the  service  from  which  it  is 
receiving  its  current,  and  the  pressure  available  for  driving  current 
through  the  coils   of  the   machine   is   the  difference  between   this 
back  pressure  and  the  pressure  of  supply.     It  may  be  taken  that  the 
back  pressure  multiplied  by  the  current  measures  the  useful  work, 
and  the  difference  between  the  back  pressure  and  that  of  supply, 
multiplied  by  the  current,  represents  the  charge  made  for  conversion. 
In  the  shunt-wound  motor  the  full  pressure  of  the  service  is  delivered 
to  the  field  coils,  and  only  the  small  pressure  necessary  for  driving  the 
current  through  its   coils,  to   the  armature.      When  the  armature 
increases  its  speed,  as  when  the  load  is  lightened,  and  thereby  increases 
the  back  pressure,  the  pressure  available  for  driving  current  through 
its  own  coils  is  lessened,  and  the  current  passing  through  them  is  also 
lessened,  and  with  it  the  work  the  motor  is  doing.     When  the  load  is 
increased,  the  armature  slightly  slows,  the  back  pressure  is  slightly 
reduced,  more  current  passes  through  the-  armature  coils,  and  more 
work  is  done.     The  pressure  at  the  terminals  of  the  field  coils  is 
raised  and  lowered  when  the  load  decreases  and  increases,  but  the 
difference  created  by  this  in  the  power  converted  by  the  motor  is  very 
small  compared  to  the  difference  created  by  the  decrease  or  increase 
of  current  in  the  armature  coils.     The  lower  the  resistance  of  the 
armature  coils,  the  more  nearly  constant  is  the  speed  of  the  shunt- 
wound  motor  with  varying  load,  because  the  charge  for  the  passage  of 
the  increased  current  is  less,  and  vice  versa. 

The  compound-wound  motor  is  made  in  two  forms.  The  series 
coils  may  be  connected  to  aid  the  shunt  coils,  or  to  oppose  them. 
The  compound-wound  motor  is  not  much  used,  except  where  the 
series  coils  are  employed  to  give  an  increased  torque  when  the 
motor  starts,  and  they  are  often  cut  out  afterwards.  The  shunt- 
wound  motor  is  weak  in  the  matter  of  starting  torque,  and  for  the 
reason  that  when  the  motor  first  starts  from  rest,  the  back  pressure 
is  small,  and  consequently  the  current  passing  through  the  supply 
wires  is  large,  and  the  pressure  at  the  terminals  of  the  shunt  field 
coils  is  low.  Hence  the  strength  of  the  field  is  lower  than  it  is  when 
the  motor  is  running  normally,  and  very  much  lower  than  that  of 
the  series-wound  motor. 

With  all  continuous-current  motors  it  is  necessary  to  provide 
starting  gear,  consisting  of  a  resistance  which  is  inserted  in  the 


276 


ELECTRICITY   IN   MINING 


PuSCfc 


armature  and  field  circuit  in  the  case  of  the  series-wound  motor,  and 
in  the  armature  circuit  alone  in  the  case  of  the  shunt-wound  motor. 

Figs.  116  and  117  are 
diagrams  of  connec- 
tions for  starting  series 
and  shunt- wound  gen- 
erators. Figs.  118  to 
122  are  diagrams  of 
connections  of  various 
motor-starting  appa- 
ratus. The  starting 
gear  consists  of  a 
^cr.  resistance  which  may 
be  of  metal,  such  as 
a  wire  or  a  strip  coiled 
Armature  into  different  forms, 

FIG.  119.— Diagram  of  Connections  of  the  B.  T.  H.  Co.'s  and.  ^el(1  in  anv  con- 
Form  A  Rheostat  for  starting  Shunt-wound  Motors,  venient  arrangement, 
with  no  Voltage  Release.  When  the  Pressure  falls  guch  ag  inside  a  per- 
below  a  certain  Figure,  the  Electro  Magnet  releases  «  ,  ,  ,  \, 

the  Contact  Bar.  lorated  metal  box,  the 

object  being  to  allow 

free  passage  of  air  to  the  surface  of  the  conductive  resistance.     The 

resistance  is  also  made 
in  the  form  of  a  liquid, 
such  as  sulphate  of 
soda,  held  in  an  iron 
trough  with  an  iron 
wedge-shaped  plunger 
arranged  to  be  pushed 
into  the  liquid.  In 
either  case  when  the 
motor  is  started,  the 
full  available  resist- 
ance is  thrown  into 
circuit,  and  it  is  gradu- 
ally cut  out,  section 

FIG.  120.— Connections  of  the  B.  T.  H.  Co.'s  Form  A  y  ,  Sectl0n>     as 

Starting  Rheostat  for  Shunt-wound  Motors,  with  no  motor  gets  UP  speed. 

Voltage  and  Overload  Release.     The  Handle  shown  on  In    the    case    of    the 

the  Left  is  held  against  the  Upper  Electro  Magnet  on  gnunt  -  wound    motor 

the  Right  when  the  Motor  is  running,  and  is  released  , , 

by  the  Coils  of  the  Electro  Magnet  being  short  circuited  tn^     current    IS    nrst 

in  either  Event,  Overload  or  no  Pressure.  switched  on  the  field 

coils,  and  then  after  a 

short  interval  it  is  switched  on  to  the  armature  coils,  through  the  resist- 
ance.    Where  a  liquid  resistance  is  employed,  it  is  gradually  cut  out 


DRIVING   MACHINES   BY  ELECTRICITY         277 


by  forcing  the  plunger  more  and  more  into  the  liquid,  till  it  finally 
makes  contact  with 
the  trough  containing 
the  liquid,  at  the 
bottom.  In  a  form 
of  liquid-starting  re- 
sistance which  has 
been  developed  on  the 
Continent,  in  the  first 
instance  for  electric 
railway  locomotives, 
and  later  for  mining 
work,  the  metal  elec- 
trodes are  fixed  per- 
manently inside  of 
containing  vessels, 
and  the  liquid  is 
forced  up  by  com- 
pressed air  through  a 
hole  in  the  bottom  of 
the  containing  vessel, 
the  height  to  which 
the  liquid  rises  vary- 
ing the  resistance 
offered,  in  the  same 

manner  as  with  the  distance  to  which  the  wedge-shaped  electrode 
is  immersed.  One 
form  of  liquid-starting 
resistance  is  shown  in 
Plate  2lA.  A.n  im- 
portant point  in  con- 
nection with  all  elec- 
tric motors  is,  they 
must  not  be  started 
too  quickly ;  that  is 
to  say,  the  resistance 
must  not  be  switched 
out  too  quickly.  The 
reason  is,  in  addition 
to  the  fact  that  a 
certain  time  is  re- 

ture    to    start    from 


Switch 


FIG.  121. — Diagram  of  Connections  of  the  B.  T.  H.  Co.'s 
Form  B  Starting  Rheostat  for  Shunt-wound  Motors, 
with  Overload  and  no  Load  Release.  The  Contact 
Lever,  it  will  be  noticed,  is  worked  by  the  Worm  and 
Wheel  shown  instead  of  directly  by  Hand,  as  in 
Form  A.  The  Contact  Bar  is  held  by  Magnetic 
Attraction  to  the  Electro  Magnet  on  the  Left  when 
the  Motor  is  running,  and  is  released  by  the  Magnet 
Coils  being  short  circuited  in  Case  of  Overload,  and 
directly  weakened  with  no  Voltage. 


Armature 


FIG.  122. — Diagram  of  Connections  of  the  B.  T.  H.  Co.'s 
Form  B  Ehe°ostat  for  starting  shunt-wound  Motors 

with  Overload  Release  only. 

rest,  when  a  powerful 

current  is  passing  through  the   armature  coils,  the  field  which  it 


278 


ELECTRICITY   IN   MINING 


creates  tends  to  neutralize  and  to  overpower  the  field  created  by 
the  field  magnet  coils,  with  the  result  that  it  is  possible  to  have 
conditions  under  which  the  motor  cannot  start,  because  there  is 
no  appreciable  field  to  create  motion  in  the  armature  conductors. 
There  are  several  forms  of  apparatus  on  the  market  in  which  the 
attendant  is  not  allowed  to  switch  on  too  quickly.  In  particular, 
he  is  obliged  to  dwell  a  certain  time  on  the  first  stop,  so  that  the 
field  magnets  in  the  case  of  the  shunt-wound  motor  may  become 
thoroughly  energized.  Messrs.  Keyrolle  have  worked  out  a  starting 


_~ILV_Tr"L~_~ 


PIG,  123.— Section  of  Messrs.  Keyrolle's  Packing  Ring  for  Gas-tight  Motor  Starters. 

resistance  shown  in  Plates  20A  and  20B,  in  which  the  passage  of  the 
current  lowers  the  resistance.  Fig.  123  shows  the  arrangement 
adopted  by  this  firm  for  preventing  gas  penetrating  to  the  starting 
contacts.  Plates  2lB  and  22A  and  22B  show  motor-starting  panels  for 
use  in  mines,  and  Plate  21  c  an  oil-enclosed  switch  for  use  in  bye. 

The  three-phase  motor  is  very  similar  to  the  continuous-current 
motor  in  appearance,  and  it  is  very  similar  to  the  three-phase 
generator  in  many  respects.  There  is  the  same  iron  or  steel  contain- 
ing cylinder  as  in  the  majority  of  continuous-current  motors,  but  in 


DRIVING   MACHINES   BY   ELECTRICITY         279 

place  of  the  field  poles  extending  radially  inwards  of  the  continuous- 
current  motor,  there  are  the  slotted  discs  held  on  the  inside  of  the 
cylinder,  just  as  in  the  armature  of  the  three-phase  generator  with 
rotating  field  magnets.  The  slots  in  the  discs  accommodate  the 
windings  of  the  two  or  three  sets  of  coils,  according  as  the  motor  is 
arranged  to  work  with  two-  or  three-phase  currents,  and  the  cylinder 
with  its  discs  and  coils  is  known  as  the  "stator."  The  "rotor,"  as 
the  moving  member  of  the  two-  and  three-phase  motor  is  called, 
corresponds  to  the  armature  of  the  continuous-current  motor,  and  is 
very  similar  to  it  up  to  a  certain  point.  There  are  the  same  slotted 
discs  on  a  spider  sleeve  keyed  on  the  rotating  shaft,  and  the  coils  are 
wound  or  fixed  in  the  slots  as  in  the  continuous-current  armature. 
But  it  has  no  commutator,  and  in  this  respect  is  simpler  than  the 
continuous-current  motor,  and  less  liable  to  get  out  of  order,  the 
commutator  being  one  of  the  great  sources  of  trouble,  especially  where 
a  motor  is  employed  under  conditions  such  as  those  that  rule  in 
mining  work.  There  are  two  forms  of  rotor,  known  as  the  "  squirrel 
cage  "  and  the  "  wound  "  rotor. 

The  squirrel-cage  rotor  has  conductors  embedded  in  the  slots  in 
the  periphery  of  the  iron  core,  the  ends  of  the  conductors  at  both  ends 
of  the  core  being  joined  by  circles  of  copper,  the  whole  forming  an 
apparatus  very  similar  to  the  cage  that  squirrels  are  made  to  perform 
in.  In  the  wound  rotor  the  coils  are  wound  in  two  or  three  sets, 
according  as  the  machine  is  for  two  or  three  phases,  and  they  are  fixed 
in  the  slots  very  much  in  the  same  way  as  the  coils  of  a  continuous 
current  motor ;  but  it  is  arranged  that  when  the  motor  is  being  started, 
the  ends  of  the  coils  are  brought  out  to  slip  rings  on  the  rotor  shaft, 
against  which  brushes  bear,  adjustable  resistances  being  connected  to 
the  brushes.  The  squirrel-cage  rotor  is  started  either  by  simply 
switching  the  stator  coils  on  to  the  supply  service  directly,  or  by  using 
an  "  auto-transformer,"  an  apparatus  which  transforms  the  pressure  of 
supply  down  to  a  low  figure,  so  that  the  currents  passing  in  the  stator 
coils  and  the  currents  induced  in  the  rotor  coils  are  small,  until  the 
rotor  has  got  up  speed,  when  the  full  pressure  of  supply  is  switched 
on.  An  auto-transformer  is  merely  a  small  transformer  enclosed  in 
a  box  which  may  be  fixed  in  any  convenient  position,  and  which 
has  a  double  throw  switch  on  the  top,  one  set  of  contacts  connect- 
ing the  low  pressure,  the  other  set  the  full  service. 

With  wound  rotors,  the  motor  is  started  very  much  in  the  same 
way  as  the  shunt-wound  continuous-current  motor.  The  supply 
pressure  is  switched  on  to  the  stator  coils,  and  at  the  same  time 
the  full  resistance  is  connected  to  the  rotor  coils,  and  is  gradually 
switched  out  as  the  rotor  gets  up  speed,  the  rotor  coils  finally  being 
short  circuited,  in  a  similar  manner  to  those  of  the  squirrel  cage. 
Fig.  124  shows  the  connections  for  this,  and  Fig.  125  is  a  diagram  of 


280  ELECTRICITY  IN   MINING 

the  connections  of  a  Westinghouse  three-phase  motor  starter.  The 
reason  for  employing  the  auto-transformer,  and  the  resistance  in  the 
case  of  the  wound  rotor,  is  that  which  was  given  for  the  continuous- 
current  motor.  If  the  full  current  is  allowed  to  be  induced  in  the 
rotor  coils  that  would  be  induced,  when  it  starts  from  rest  if  the 
full  pressure  were  applied  to  the  stator  coils,  the  magnetic  fields 
created  by  the  current  in  the  coils  of  the  rotor  would  overcome 
the  fields  created  by  the  currents  in  the  stator  coils.  The  behaviour 
of  the  two-  and  three-phase  "  induction  "  motor,  as  it  is  called,  or 
asynchronous  motor,  is  very  similar  in  almost  every  respect  to  that 
of  the  shunt-wound  motor,  though  the  reason  for  its  behaviour  is 
different. 

In  the  induction  motor  the  field  created  by  the  currents  in  the 
stator  coils  is  not  stationary  within  the  cylindrical  space  occupied  by 
the  rotor,  but  is  continually  moving  around  it,  as  the  currents  in  the 

SLIP  RINGS  STARTING  RESISTANCE 

SWITCHES 


THREE  PHASE  WOUND 
ROTOR  Cbtus,  CONNECTED 
TO  SUP  RINGS  ON  SHAFT 

FIG.  124. — Diagram  of  Connections  for  starting  Three-phase  Motors  with 
Wound  Eotors. 

different  phases  rise  and  fall  and  reverse,  and  the  variations  in  the 
currents  in  the  stator  coils  induce  currents  in  the  rotor  coils  in  such 
directions,  that  the  coils  and  the  iron  core  to  which  they  are  attached 
move  round  after  the  magnetic  field  created  by  the  stator  coils.  The 
rotor  never  attains  the  same  speed  as  the  stator  coils  it  is  moving  after, 
just  as  the  armature  of  the  shunt-wound  motor  never  creates  a  back 
pressure  equal  to  the  supply  pressure.  If  the  back  pressure  of  a  con- 
tinuous-current motor  equalled  the  supply  pressure,  no  current  would 
pass,  and  the  efficiency  of  the  system  would  be  100  per  cent.  Simi- 
larly, if  the  speed  of  the  rotor  equalled  that  of  the  revolving  field 
created  by  the  stator  currents,  no  current  would  pass,  and  the  efficiency 
of  the  system  would  again  be  100  per  cent.  But  no  motor  is  without 
friction,  and  all  motors  make  a  charge  upon  the  energy  delivered  to 
them  for  creating  the  magnetic  field,  hence  there  is  always  a  differ- 
ence between  the  supply  and  back  pressures  in  continuous-current 
machines,  and  between  the  speed  of  the  field  and  that  of  the  rotor  in 


I'***  .?•?'§  ^ 


PLATE  20A. — Messrs.  Reyrolle's  Start- 
ing Switch  and  Resistance  complete 
in  iron  case.  The  Motor  is  started 
by  turning  the  Wheel  at  the  Top. 


PLATE  20B. — Internal  Construction  of 
Messrs.  Reyrolle's  Starting  Switch 
and  Resistance.  The  Resistance  is 
lowered  by  the  Passage  of  the  Cur- 
rent through  it,  hence  the  Current 
automatically  increases. 


PLATE  20c. — Reversing  Apparatus,  for  Electric  Winding,  made  by  the 
International  Electrical  Engineering  Co.,  of  Liege.  The  View  shows  the 
Enside  of  the  Case  in  which  the  Connections  are  made. 


[To  face  p.  280. 


DRIVING   MACHINES   BY   ELECTRICITY         281 


Junction  Box  for 
Incoming  Cable 


4  -""  ^        '  « 


' !       !    rriti 

I  fc-—L  h-k-4.4— 4 

%.  .  .  _Tl    I        I     L 


Junction  Box  for 
Cable  to  Motor 


>— <o 

FIG.  125.— Diagram  showing  Connections  and  Arrangement  of  Westinghouse 
Three-phase  Motor  Starting  Box. 


282  ELECTRICITY   IN    MINING 

induction  motors.  Further,  just  as  the  current  required  to  turn  the 
armature  of  the  continuous-current  motor  increases  or  decreases 
according  as  the  speed  of  the  armature  falls  or  rises,  so  the  currents 
induced  in  the  rotor  of  the  induction  motor  increase  or  decrease  as 
the  speed  of  the  rotor  decreases  or  increases,  the  speed  decreasing  with 
increased  load,  and  increasing  with  decreasing  load.  The  induction 
motor  is  practically  self-governing  within  its  own  limits,  just  as  the 
shunt- wound  motor  is.  An  increased  load  causes  the  motor  to 
slightly  slow  up,  this  allowing  the  necessary  increased  current  to  be 
induced  in  the  rotor  coils.  The  induction  motor  is  really  a  rotary 
transformer,  the  stator  coils  being  the  primary,  and  the  rotor  coils 
the  secondary,  and  just  as  in  the  ordinary  stationary  transformer  the 
currents  in  the  primary  induce  pressures  in  the  secondary,  so  the 
currents  in  the  secondary  induce  currents  in  the  primary.  Plates 
19fi,  19c,  and  19D  show  a  complete  three-phase  motor,  and  its 
"  stator  "  and  "  wound  "  rotor ;  and  Plate  22c  shows  a  "  wound  " 
rotor,  with  slip  rings  apart  from  the  bobbin. 


Methods  of  varying  the  Speed  of  Electric  Motors 

With  continuous-current  motors  there  are  two  methods  of  varying 
the  speed,  by  varying  the  pressure  delivered  to  the  terminals  of  the 


wwvwi 


FIELD  COILS  SUPPLY  CABLES 


RHEOSTAT 


PIG.  126. — Diagram  of  Connections  for  regulating  the  Speed  of  a  Series-wound 
Motor  by  varying  the  Pressure.  The  Resistance  absorbs  more  or  less  of 
the  Pressure  of  Supply.  It  will  be  noticed  that  the  Connections  are  similar 
to  those  for  starting  a  Series-wound  Motor.  It  is  the  Resistance  that 
must  be  different. 

motor,  and  by  varying  the  strength  of  the  current  passing  through  the 
field  magnets.     With  the  series-wound  motor,  the  most  convenient 


DRIVING   MACHINES   BY   ELECTRICITY         283 

method  is  by  varying  the  pressure  delivered  to  the  motor,  and  this  is 
done  by  inserting  a  resistance  between  the  positive  supply  cable 
and  the  positive  terminal  of  the  motor,  the  resistance  being  divided 
into  sections  similar  to  that  of  the  starting  resistance,  and  more 
or  less  of  it  being  thrown  into,  or  cut  out  of,  the  circuit  by  means 
of  a  switch.  A  diagram  of  this  is  shown  in  Fig.  126.  One  im- 
portant caution  is  necessary  here,  the  neglect  of  which  has  led  to 
trouble  in  the  past.  The  resistance  which  is  arranged  to  be  thrown 
into  the  circuit  of  a  motor  on  starting  is  not  designed  to  carry  the 
current  the  motor  will  use  when  doing  its  work.  In  the  case  of 
the  starting  resistance,  the  current  is  only  allowed  to  pass  through 
the  conductor  or  the  liquid  for  the  very  short  period  occupied 
in  starting,  and  as  the  time  factor  is  of  enormous  importance 
in  this  case,  the  heat  liberated  by  the  current  is  so  small  com- 
paratively, while  with  a  properly  designed  resistance  the  heat 
passing  away  from  the  resistance  is  so  large  comparatively,  that  the 
temperature  of  the  apparatus  should  not  be  appreciably  increased. 
For  this  reason  very  much  smaller  metallic  or  liquid  resistances  can 
be  employed  than  would  be  possible  if  the  current  is  allowed  to 
pass  through  it  continually.  Where  a  resistance  is  employed  to  vary 
the  speed  of  the  series- wound  motor,  therefore,  by  varying  the 
pressure,  that  is  by  using  up  a  portion  of  the  pressure  that  would  be 
delivered  to  the  motor,  the  resistance  must  be  calculated  of  such  a 
section,  whether  liquid  or  metallic,  and  the  means  of  dissipating 
the  heat  generated  must  be  such  that  the  rise  in  temperature  does 
not  exceed  that  of  a  generator  or  motor  when  in  regular  use.  If 
this  condition  is  not  observed,  trouble  will  arise  with  the  resistances. 
With  metallic  resistance  any  joints  there  may  be — there  should  be 
no  joint  in  the  resistance  at  all,  if  possible — and  the  connections  to 
the  stops  forming  the  sections  of  the  resistance,  will  be  very  likely 
to  become  disconnected,  arcing  following,  and  trouble  generally. 
With  liquid  resistance  evaporation  always  takes  place,  both  when  the 
current  is  passing  and  when  it  is  not.  The  evaporation  will  be 
greater  than  when  the  current  is  passing,  because  of  the  heat 
delivered  to  the  liquid  and  the  higher  temperature  to  which  the 
liquid  is  raised,  that  is  to  say,  the  greater  the  current  that  is  allowed 
to  pass  through  the  liquid  the  greater  will  be  the  rate  of  evaporation. 
And  this  leads  to  two  forms  of  trouble;  not  only  does  the  vessel 
containing  the  liquid  require  very  frequent  replenishment  with  water, 
but  the  liquid  itself  changes  its  physical  properties.  Its  resistance 
will  change,  and  the  result  of  interposing  a  smaller  or  larger  quantity 
in  the  circuit  will  also  be  changed.  It  is,  however,  perfectly 
practicable  to  provide  either  a  metallic  or  a  liquid  resistance  that 
shall  answer  all  the  conditions  required  for  regulating  the  speed 
of  a  series-wound  motor,  but  though  the  plan  is  a  convenient  one, 


284  ELECTRICITY  IN   MINING 

providing  the  resistance  is  properly  arranged,  it  is  a  very  wasteful 
one  if  the  motor  is  working  for  any  period  at  anything  much  less 
than  its  full  load,  since  the  whole  of  the  electrical  energy  that 
is  converted  into  heat  in  the  controlling  resistance  is  absolutely 
wasted,  and  tends  to  raise  the  temperature  of  the  apparatus 
generally. 

The  other  method  with  a  series-wound  motor  is,  by  providing  a 
shunt  to  the  field  coils,  dividing  the  shunt  into  sections,  just  as  with 
the  starting  resistance  and  the  series  controlling  resistance,  and  shunt- 
ing the  field  coils  by  a  greater  or  less  portion  of  the  shunt,  according 
as  the  speed  of  the  motor  is  required  to  go  down  or  up,  as  shown 
diagrammatically  in  Fig.  127.  It  should,  perhaps,  be  mentioned 


M/VWWV 

F"ieri  t*\   fVMi  «. 


SUPPLY  CABLES. 


RHEOSTAT 


FIG.  127. — Diagram  of  Connections  for  varying  the  Speed  of  a  Series- wound 
Motor  by  varying  the  Field  Current.  The  Besistance  shown  shunts 
more  or  less  of  the  Normal  Current. 

that  lowering  the  pressure  delivered  to  the  terminals  of  the  series- 
wound  or  shunt-wound  motor  by  the  insertion  of  resistance  in  the 
main  circuit,  lowers  the  speed  of  the  motor,  and  that  shunting  the 
field  coils  of  the  series-wound  motor,  lessening  the  current  passing 
through  the  field  coils,  raises  the  speed  of  the  motor. 

With  a  shunt-wound  motor  a  variable  resistance  may  be  fixed  in 
the  main  circuit,  or  preferably  in  the  armature  circuit  only,  just  as 
with  the  series-wound  motor,  but  the  more  common  arrangement  is, 
a  variable  resistance  is  inserted  in  the  circuit  of  the  field  coils,  as 
shown  in  Fig.  128.  Increasing  this  resistance  increases  the  speed  of 
the  motor,  and  vice  versa.  It  will  be  seen  that  the  method  of  lowering 
the  field  current  is  preferable  to  that  of  lowering  the  pressure  in  the 
main  circuit,  inasmuch  as  though  there  is  a  certain  waste  in  the 


DRIVING   MACHINES   BY  ELECTRICITY         285 

resistance,  the  waste  is  very  much  smaller  than  where  the  resistance 
is  added  to  the  main  circuit.  There  is  a  limit,  however,  to  the 
application  of  the  method  of  adding  resistance  to  the  circuit  of  the 
field  coils  of  the  shunt-wound  motor.  As  the  field  is  weakened, 
and  especially  if,  as  is  required  in  many  cases,  the  current  through 


SUPPLY  CABLES 
Fiexo  COILS 


RHEOSTAT 


FIG.  128.— Diagram  of  Connections  for  regulating  the  Speed  of  a  Shunt- 
wound  Motor  by  varying  the  Field  Current. 

the  armature  is  increased,  so  that  additional  work  may  be  done  by 
the  motor,  sparking  commences  at  the  brushes,  and  increases,  not- 
withstanding what  can  be  done  by  the  brush  rocker,  until  at  about 
25  per  cent,  above  the  normal  speed  no  further  increase  is  possible. 


The  Motor  with  Commutating  Poles 

The  trouble,  however,  mentioned  above  has  been  completely 
overcome  by  what  is  known  as  the  dynamo  with  commutating  poles, 
described  in  Chapter  IV.  Commutating  poles  are  additional  field 
magnet  poles,  smaller  than  the  proper  poles  of  the  machine,  and 
fixed  between  them  in  such  a  position  that  they  neutralize  the 
current  generated  in  the  coil  passing  under  the  brush  at  the  moment 
of  commutation.  By  their  aid  the  speed  of  shunt-wound  motors 
may  be  varied  in  the  rates  of  one  to  six,  by  varying  the  current 
passing  in  the  field  coils,  and  without  sparking.  This  method  is 
being  used  very  largely  for  driving  machine  tools  and  other 
apparatus,  where  large  variations  of  speed  are  required. 


286  ELECTRICITY  IN    MINING 


Varying  the  Speed  of  the  Three-phase  Motor 

The  speed  of  the  three-phase  motor,  as  explained  above,  is  con- 
trolled by  the  speed  of  the  revolving  field,  and  this  again  is  the 
periodicity  of  the  service.  Hence,  one  method  of  varying  the  speed 
of  a  three-phase  motor  is  by  varying  the  number  of  poles  of  the 
stator,  and  thereby  varying  the  speed  of  the  revolving  field,  and  with 
it  the  speed  of  the  motor  that  is  moving  after  it.  The  range,  how- 
ever, of  variation  of  speed  by  this  method,  it  will  be  seen,  is  small. 
With  small  machines,  for  instance,  having  six  or  eight  poles  in  the 
stator,  cutting  out  half  reduces  the  speed  one-half,  and  so  on.  The 
method  more  frequently  adopted  is  by  adding  a  resistance  to  the 
rotor  circuit,  similar  to  that  arranged  for  starting  the  wound  rotor, 
and  making  this  resistance  sufficiently  large,  etc.,  to  be  allowed  to 
remain  in  circuit  for  any  period  that  may  be  desired  when  the  motor 
is  at  work.  It  need  hardly  be  said  that  this  method  is  wasteful,  but 
in  certain  cases,  as  will  be  described,  it  is,  perhaps,  the  best  available. 
There  are  signs  that  apparatus  for  varying  the  frequency  of  a  service 
are  being  worked  out,  and  probably  some  arrangement  of  the  kind 
will  be  adopted  for  controlling  the  speed  of  three-phase  motors. 


Electrically  Driven  Pumps 

Driving  pumps  by  means  of  electric  motors  was  the  earliest 
application  of  electricity  to  power  purposes  in  mines,  the  first  having 
been  at  Trafalgar  Colliery  in  the  Forest  of  Dean,  by  the  late  Mr. 
William  Blanch  Brain.  Mr.  Brain  had  a  good  deal  of  water  in  some 
of  his  workings,  at  some  distance  from  the  pit-bottom,  and  he  had 
been  driving  a  pump  by  steam  taken  down  the  pit.  He  fixed  a 
Siemen's  machine  of  those  days  that  was  capable  of  furnishing  one 
arc  lamp,  to  drive  the  pump  by  means  of  a  belt,  and  he  fixed  an 
"  A  "  gramme  machine,  the  1500  to  2000  watt  machine  of  those  days, 
in  an  engine  house  on  the  bank,  and  drove  it  by  means  of  a  single 
cylinder  engine.  Both  generator  and  motor  were  series  wound.  The 
arrangement  answered  remarkably  well,  and  the  quantity  of  coal 
saved  was  something  very  considerable.  In  addition  to  the  cables 
connecting  the  two  machines,  there  was  also  a  telephone  fixed  in  the 
engine  house,  with  a  battery  and  a  pair  of  wires  which  were  led  to 
the  pump  house,  connected  to  a  moving  contact  which  completed  the 
circuit  at  every  stroke  of  the  pump,  so  that  the  attendant  in  the 
engine  house  could  hear  if  the  pump  was  working  satisfactorily. 
Pumping  is  a  particularly  suitable  class  of  work  for  electric  driving, 
for  the  principal  reason  that  the  pumps  are  very  often  required  to 


DRIVING   MACHINES   BY   ELECTRICITY         287 

be  at  some  distance  from  the  shaft  bottom,  and  in  out-of-the-way 
positions ;  and  it  is  much  easier  to  run  a  pair  of  cables  to  these 
positions,  than  either  a  steam,  compressed  air,  or  hydraulic  pipe. 
Where  dip  pumps  also  are  employed,  the  arrangement  of  the  pump 
fixed  on  a  trolley  with  its  motor,  gearing,  and  starting  switch,  the 
trolley  being  mounted  on  wheels  to  run  on  the  mine  roads,  is  very 
convenient,  as  it  is  easy  to  provide  a  sufficient  length  of  cable  on 
drums  fixed  on  the  trolley,  to  enable  the  pump  to  follow  the  water 
right  down  without  making  joints,  and  without  any  change  in  the 
resistance  of  the  leads.  Electricity  has,  however,  been  applied  to 
pumping  in  mines  under  every  condition  where  pumping  is  required. 
It  is  employed  in  sinking  pumps,  in  driving  pumps  at  the  bottom  of 
the  shaft  when  the  water  from  the  workings  has  been  delivered  to  a 
sump  there,  for  dip  workings,  for  pumping  water  from  rivers,  for 
boiler  feed,  etc. 


Forms  of  Pumps 

There  are  three  forms  of  pumps  employed  in  mines,  centrifugal 
pumps,  ram  or  plunger  pumps,  and  bucket  pumps.  Of  these  the 
ram  pump  in  its  three-throw  form  is  the  one  most  commonly 
employed ;  but  since  the  improvement  that  has  taken  place  within 
recent  years  in  the  centrifugal  pump,  this  is  also  making  way.  The 
bucket  pump  is  only  employed,  so  far  as  the  author  is  aware,  for 
pumping  from  sumps  or  lodges  in  the  mine  shaft.  The  centrifugal 
pump  is  the  opposite  of  the  water  turbine,  with  certain  modifications. 
In  the  water  turbine  there  are  a  number  of  blades  arranged  around 
a  shaft,  and  the  water  impinging  upon  them  turns  the  shaft,  to 
which  they  are  attached.  In  the  centrifugal  pump  there  are  again 
a  number  of  blades  surrounding  a  shaft,  and  when  the  shaft  and  its 
blades  are  revolved  by  mechanical  power  from  outside,  the  water 
which  is  made  to  enter  the  pump  at  the  centre  is  forced  outwards 
by  the  action  of  the  blades,  and  by  its  own  centrifugal  force,  and  is 
driven  into  the  delivery  pipe.  The  earlier  forms  of  centrifugal  pump 
were  only  available  for  very  low  lifts,  50'  being  considered  high, 
and  while  the  efficiencies  were  comparatively  high  with  very  low 
lifts,  they  fell  very  quickly  if  the  lift  was  increased.  Modern 
centrifugal  pumps,  however,  deal  with  lifts  as  much  as  2000',  and  it 
is  claimed  that  the  efficiencies  are  higher  than  those  of  the  ram 
pump.  Improvement  in  the  efficiency  of  the  centrifugal  pump  has 
been  obtained  by  a  careful  study  of  the  course  of  the  water  in  the 
pump.  Power  is  wasted  in  every  pump  by  eddies  that  are  created 
in  the  water,  and  by  a  study  of  the  form  which  these  eddies  take, 
and  by  designing  the  pump  so  that  no  eddies  are  made,  the  efficiency 


288  ELECTRICITY  IN   MINING 

has  been  increased.  One  great  trouble  in  the  centrifugal  pump  which 
led  to  its  previous  inefficiency  was,  the  water  in  passing  through  the 
pump  was  going  at  a  very  high  velocity,  and  when  delivered  from 
the  pump  into  the  rising  main,  it  met  and  had  to  be  absorbed  by  a 
column  of  water  which  was  moving  at  a  very  much  slower  rate,  and 
this  led  to  the  formation  of  a  number  of  eddies,  and  back  pressures, 
opposing  the  onward  motion  of  the  water,  and  absorbing  a  part  of  the 
power  that  was  being  delivered  to  the  pump  shaft. 

Pump  makers  express  the  fact  sometimes  by  saying  that  the 
difficulty  is  in  converting  velocity  head  into  pressure  head.  They 
mean  what  the  author  has  expressed  above.  The  water  when  in 
rapid  motion  is  possessed  of  a  certain  quantity  of  energy  in  virtue 
of  its  velocity.  It  is  known  as  kinetic  energy.  When  it  joins 
the  slowly  moving  column,  it  does  not  lose  the  energy  that  was 
imparted  to  it,  except  in  so  far  as  it  may  be  deprived  of  a  portion 
of  it  by  the  eddies  and  back  pressures  that  are  formed ;  and  what 
is  really  required  is,  the  conversion  of  the  high  rate  of  motion  to  the 
slow  rate  of  motion,  without  loss  of  energy.  In  the  older  form  of 
centrifugal  pump,  the  water  when  leaving  the  fan  blades  was  delivered 
into  a  whirling  chamber,  where  it  was  at  liberty  to  form  as  many 
eddies  as  it  pleased,  and  from  which  it  was  finally  pushed  out  by 
the  pressure  of  the  water  behind  it,  but  with  a  considerable  expen- 
diture of  unnecessary  power.  In  the  modern  centrifugal  pump  the 
water  is  guided  after  it  leaves  the  fan  blades  by  various  devices, 
by  guide  vanes  in  the  Worthington  turbine  pump  (Fig.  129  shows 
a  section  of  the  Worthington  Co.'s  multistage  centrifugal  pump) ;  by  a 
guide  ring  in  the  Mather  &  Platt  centrifugal  pump ;  and  by  other 
arrangements,  all  very  similar,  by  other  makers.  In  all  cases  the 
object  to  be  attained  is  the  avoidance  of  shocks,  the  avoidance  of  the 
impingement  of  the  water  against  dead  surfaces  of  metal  or  water, 
and  the  gentle  guidance  of  the  water  by  curved  passages,  carefully 
calculated  for  the  purpose,  so  that  its  velocity  is  gradually  lost, 
and  when  it  joins  the  rising  column,  the  energy  with  which  it  left 
the  vanes  has  been  converted  to  the  form  in  which  it  will  best  assist 
to  push  the  column  above  it  upwards. 

Centrifugal  pumps  are  now  made  in  three  forms,  for  very  low 
lifts  up  to  30',  for  medium  lifts  up  to  70',  and  for  high  lifts  up  to 
100  and  odd  feet,  and  any  of  these  forms  may  be  connected  together 
in  series,  their  lifts  then  being  added  together.  The  speeds  of  the 
pumps  for  the  different  lifts  do  not  differ  very  much.  With  each 
form  of  pump,  and  with  each  size,  there  is  a  certain  speed  at  which 
the  highest  efficiency  is  obtained,  there  is  a  certain  quantity  for  the 
highest  efficiency,  and  a  certain  head.  Larger  quantities  may  be 
delivered  by  any  given  pump  by  driving  it  at  a  higher  speed.  Any 
given  quantity  may  be  raised  to  a  greater  height,  also  by  increasing 


a1     £ 

'3 


o  o 


DRIVING   MACHINES   BY   ELECTRICITY         289 


the   speed,   but   the  efficiency  goes  down   very   quickly  after  the 
quantity  is  reached  for  which  the  pump  is  constructed.     Figs.  130 

u 


290 


ELECTRICITY   IN   MINING 


and  131,  which  are  curves  taken  by  Messrs.  Mather  &  Platt  from  a 
single  chamber  pump,  constructed  for  the  Newcastle  Corporation 
Electricity  Works,  to  deliver  1250  gallons  of  water  per  minute  against 
a  head  of  from  100'  to  117',  the  pump  running  at  700  revolutions  per 
minute,  show  these  points  very  clearly.  It  will  be  seen  from  Fig. 
130,  in  which  the  quantity  pumped  was  maintained  constant,  that 
the  efficiency  rises  very  quickly  from  nothing,  as  the  revolutions  of 
the  pump  increase,  till  at  650  revolutions  the  increase  of  efficiency 
is  very  gradual.  From  700  to  750  revolutions  there  is  practically  no 
change.  At  800  revolutions  the  efficiency  is  slightly  lower,  and  after 
that  it  falls  very  quickly,  till  at  970  it  is  only  42  per  cent.,  the 


CONSTANT  QUANTITY. 


200  300 


400  SOO  600  700  800  900  1000 

Revolutions  per  Minute, 


FIG.  130. — Efficiency  Curve  of  a  Centrifugal  Pump,  made  by  Messrs.  Mather 
&  Platt,  with  Constant  Quantity  and  Varying  Speed  and  Head. 

highest  efficiency  being  about  72  per  cent.  When  running  at  con- 
stant speed,  as  shown  in  Fig.  131,  it  will  be  noticed  again  that  the 
efficiency  increases  very  rapidly  as  the  quantity  of  water  delivered 
increases,  up  to  1000  gallons  per  minute,  then  the  increase  is  slow. 
There  is  very  little  change  between  1200  and  1300  gallons.  At  1400 
gallons  it  falls  slightly,  and  afterwards  it  falls  very  quickly,  until  at 
2020  gallons  it  is  only  25  per  cent.  When  the  head  pumped  against 
is  maintained  constant,  the  speed  and  the  quantity  being  changed, 
the  speed  remains  practically  the  same,  about  680  revolutions,  until 
the  quantity  is  900  gallons  per  minute.  The  speed  and  the  quantity 
then  go  up  together,  the  speed  being  870,  when  the  quantity  is 
about  2350.  The  efficiency  rises  very  quickly,  as  before,  until  the 


DRIVING   MACHINES   BY   ELECTRICITY 


291 


quantity  is  1100  gallons  per  minute,  and  the  speed  slightly  over 
700;  it  then  rises  very  slowly,  there  being  very  little  difference 
between  1300  and  1400  gallons,  and  between  720  and  725  revolu- 
tions, it  falls  slightly  at  1600  gallons  and  745  revolutions,  and  then 
falls  very  rapidly  till  at  2300  gallons,  and  at  860  revolutions,  it  is 
only  20  per  cent.  The  moral  of  the  above  is,  that  it  is  wiser  to 
run  centrifugal  pumps  at  constant  speed  for  constant  quantity,  and 
at  the  speed  and  for  the  quantity  at  which  they  are  designed  to  give 
their  highest  efficiency.  It  is  usually  wise  to  work  most  machinery 
about  mines  at  a  lower  output  than  they  are  made  for,  because  it 
lowers  the  repairs  bill.  With  the  modern  centrifugal  pump  it  will 


CONSTANT  SPEED. 


> 


200 


\ 


X 


1000  1200          1400 

Gallons  per  Minute 


MOO 


FIG.  131. — Efficiency  Curve  of  a  Centrifugal  Pump,  made  by  Messrs.  Mather 
&  Platt,  with  Speed  Constant  and  Quantity  and  Head  varying. 

be  seen  that  the  output  can  be  lowered  something  like  25  per 
cent,  with  a  lowered  efficiency  of  only  about  4  per  cent.  Plate  23A 
shows  a  single  chamber  centrifugal  pump  driven  by  an  electric  motor. 
When  centrifugal  pumps  are  arranged  in  series,  the  delivery  of 
the  first  pump  becomes  the  suction  of  the  second,  the  delivery  of  the 
second  the  suction  of  the  third,  and  so  on,  as  many  as  twelve  pumps 
having  been  connected  in  series  in  this  way,  the  axles  of  all  the 
pumps  being  connected  together.  It  will  be  evident  that  the  cen- 
trifugal pump  lends  itself  to  electric  driving,  for  two  reasons :  The 
speed  of  the  pump  is  within  the  same  range  as  the  ordinary  speed  of 
the  electric  motor  ;  and  the  pump,  whether  single  or  in  series,  can  be 
mounted  on  a  bedplate,  on  which  room  is  left  for  the  motor,  the 


292  ELECTRICITY   IN   MINING 

axles  of  the  pumps  being  connected  mechanically  to  the  axle  of  the 
motor.  Further,  there  is  no  difficulty  in  arranging  an  electric  motor 
and  a  centrifugal  pump  vertically  one  above  the  other,  their  axles 
being  vertical,  and  the  whole  being  suspended,  say  in  a  shaft,  or  any 
other  position  where  space  or  convenience  makes  this  arrangement 
suitable.  Sinking  pumps  are  worked  on  this  plan,  as  shown  in 
Plates  24  A  and  2  5  A.  Like  the  electric  motor  itself,  the  centrifugal 
pump  is  very  convenient,  and  for  that  reason  is  suitable  for  a  great 
many  places  where  it  would  be  difficult  to  apply  either  of  the  other 
forms. 

The  motor  that  is  most  suitable  for  driving  centrifugal  pumps 
will  be  either  the  shunt-wound  continuous-current  motor,  or  the 
three-phase  motor.  Both  of  these  run  at  nearly  uniform  speed,  the 
variations  from  the  normal  speed  being  only  four  per  cent,  in 
the  case  of  the  three-phase  motor,  and  very  small  in  the  case  of 
the  shunt-wound  motor  under  ordinary  working  conditions.  The 
centrifugal  pump  would,  in  the  great  majority  of  cases,  only  be 
applied  where  a  uniform  speed  would  rule,  since  the  efficiency  of  the 
pump  is  considerably  lowered  if  it  is  run  at  much  above  or  below  its 
normal. 

Ram  Pumps 

The  ram,  or  plunger  pump,  consists  of  a  barrel  in  which  a  ram  or 
plunger  moves  to  and  fro.  The  barrel  has  inlet  and  delivery  valves, 
the  suction  stroke  of  the  plunger  opening  the  inlet  valve  and  sucking 
the  water  into  the  barrel,  the  delivery  stroke  closing  the  inlet  valve, 
opening  the  outlet,  and  driving  the  water  through  it  into  the  rising 
main.  In  mining  work  it  is  usual  to  arrange  three  pumps  with  their 
plunger  rods  on  one  crankshaft,  the  cranks  being  120°  apart,  the  crank- 
shaft being  driven  by  any  convenient  source  of  power.  For  electric 
driving  it  is  usual  to.  mount  an  electric  motor  on  the  same  carriage 
as  the  pump,  and  to  gear  it  either  directly  to  the  crankshaft  by  two 
spur  gear  wheels,  or  to  interpose  a  second  motion  shaft,  supported  on 
the  same  carriage,  driven  from  the  electric  motor  by  a  belt  or  ropes, 
the  second  motion  shaft  driving  the  pump  shaft  by  gearing.  One  of 
the  difficulties  in  connection  with  the  driving  of  the  three-throw 
pump  by  means  of  an  electric  motor  is,  the  great  reduction  that  has 
to  be  made  in  the  speed.  The  ordinary  form  of  three-throw  ram 
pump  rarely  runs  at  more  than  40  revs,  per  minute,  while  the  electric 
motor  runs  at  from  500  to  1500  revs.,  according  to  its  source. 

The  low  speed  of  the  pump  is  due  to  the  form  of  the  valves 
employed.  They  are  usually  of  the  mushroom  type,  and  each  time 
that  each  valve  opens,  the  mass  of  metal  of  which  it  is  composed  has 
to  be  moved  inwards  or  outwards  against  the  pressure  of  a  strong 


DRIVING   MACHINES   BY   ELECTRICITY         293 

spring.  If  the  pumps  run  at  a  higher  speed  than  that  mentioned,  40 
revs.,  or  thereabouts,  the  motion  of  the  valves  becomes  so  rapid,  and 
the  hammering  on  the  valve  seats  is  so  hard,  that  the  pump  is  quickly 
put  out  of  order.  Hence,  the  efficiency  of  the  ordinary  three-throw 
ram  pump  is  only  66f  per  cent,  at  its  best,  and  when  running  at  its 
highest  possible  speed,  and  from  this  has  to  be  subtracted  the 
efficiency  of  the  gearing. 

Modern  ram  pumps,  however,  have  been  very  greatly  improved, 
principally  in  Germany.  Professor  Kiedler,  who  has  investigated  the 
matter  scientifically,  has  introduced  some  considerable  improvements, 
which  have  enabled  pumps  to  be  run  at  a  very  much  higher  speed, 
and  also  their  efficiency  to  be  considerably  increased.  An  important 
feature  in  the  Eiedler  pump  is  the  shape  and  arrangement  of  the 
suction  and  delivery  valves,  as  shown  in  Fig.  132.  They  are  both 
made  comparatively  large,  and  they  are  made  to  open  mechanically, 
the  arrangement  for  opening  them  being  quite  independent  of  the 
pressure  inside  the  pump  chamber,  so  that  springs  are  entirely  dis- 
pensed with.  The  valve  also  is  opened  comparatively  widely,  so  that, 
it  is  claimed,  the  eddies  in  the  water  are  avoided.  The  speed  is 
raised  from  40  revs,  to  150  revs.,  and  the  efficiency  is  claimed  to  be 
raised  from  66f  to  90  per  cent.  It  is  usual  in  the  express  Riedler 
pump  to  have  only  one  pump  chamber,  with  one  set  of  valves,  one 
set  of  journals,  and  so  on,  the  friction  saved  in  the  lessened  number 
of  valves,  etc.,  making  up  part  of  the  increased  efficiency  of  the  pump. 
It  will  be  understood,  of  course,  that  the  pump  deals  with  a  smaller 
quantity  of  water  at  each  stroke  than  the  slow-speed  pumps.  In 
addition  to  this,  a  valve  known  as  the  "  Gutermuth,"  the  invention 
of  the  professor  of  that  name,  which  is  shown  in  Fig.  133,  has  been 
also  introduced  in  Germany,  and  to  this  country,  which  it  is  claimed 
increases  the  efficiency  of  the  ram  pump  for  two  reasons.  The  work 
of  raising  the  valve  is  considerably  lessened,  and  the  formation  of 
eddies  by  the  water  after  passing  through  the  valve  is  often,  it  is 
claimed,  almost  suppressed.  The  valve,  as  will  be  seen  from  the 
drawings,  consists  of  a  strip  of  steel  or  gun-metal,  rolled  up  into  the 
form  of  a  spiral,  as  shown,  the  end  of  the  spiral  being  placed  across 
the  valve  port,  and  the  rod  on  which  the  spiral  is  formed  being  held 
conveniently  near.  The  operation  of  opening  the  valve  consists 
simply  in  forcing  the  end  of  the  steel  or  gun-metal  plate  outwards. 
That  is  to  say,  in  the  case  of  the  suction  valve,  the  end  of  the  plate 
moves  inwards  into  the  cylinder,  and  in  the  delivery  valve  it  moves 
outward  into  the  delivery  pipe,  in  each  case  winding  the  spiral  up  a 
little  more.  It  will  be  seen  that  the  power  required  to  lift  this  form 
of  valve  may  be  much  less  than  that  required  to  lift  the  heavy  clack 
valves  usually  employed.  Further,  it  is  claimed  by  Professor  Guter- 
niuth,  that  when  the  mushroom  valve  opens,  the  water  passing  through 


294 


ELECTRICITY   IN   MINING 


the  valve  is  forced  against  the  head  of  the  valve,  and  is  broken  up, 
forming  eddies,  etc.,  beyond,  while  with  the  Gutermuth  valve  the 


DRIVING   MACHINES   BY   ELECTRICITY 


295 


water  continues  its  passage  in  a  straight  line.     The  Gutermuth  valve 
has  been  adapted  to  existing  ram  and  other  pumps,  with  the  result 


FIG.  133. — Gutermuth  ,Valves  for  Pumps.  The  Illustration  on  the  Eight 
at  the  Top  shows  how  the  Valve  is  made.  That  on  the  Left  at  the 
Bottom  shows  the  Valve  in  Position,  and  that  on  the  Bight  the  Water 
passing  through. 

that  the  travel  of  the  pump  has  been  increased  from  40  to  140  revs, 
per  minute,  and  the  quantity  of  water,  in  the  case  of  a  pump  of 
5-inch  bore  by  10-inch  stroke, 
has  been  increased  from  1500 
to  5500  gallons  per  hour,  the 
valve  space  being  increased 
from  4  square  inches  to  8| 
square  inches.  The  arrange- 
ment of  the  valves  is  shown 
in  Fig.  134. 

The  series-wound  motor 
has  been  largely  employed 
for  driving  the  ram  pump, 
mainly  because  of  its  high 
starting  torque.  To  start 
the  pump  from  rest  against 
the  pressure  of  a  column 


FIG.  134.  —  Arrangement  of  Gutermuth  Valves  on 
an    ordinary  Ram   Pump.     The    Number    of 

gmall    ^  take  ^  ^  Qi  &         Q  Muah_ 
from  the  dip  workings  to  the      room  valve. 
bottom   of    the    shaft    and 

thence  to  the  surface,  requires   a  very  considerable  starting  effort, 
though  the  power  required  when  once  the  pump  is  started  may  be 


nf     waf-or      avtPTirh*no>      Qflv 

ol    water     extending     say, 


296  ELECTRICITY  IN   MINING 

comparatively  small.  The  series-wound  motor  is  eminently  adapted 
for  that  part  of  the  work,  the  starting.  Plate  26 A  shows  an  elec- 
trically driven  three-throw  ram  pump  for  dip  workings,  and  Plate 
24B  shows  a  three-throw  electrically  driven  pump  arranged  for 
sinking.  But  there  is  another  feature  about  the  ram  pump.  The 
quantity  of  water  it  delivers  depends  upon  its  speed.  Each  stroke 
of  each  pump  delivers  a  certain  quantity  of  water,  and  therefore 
the  greater  the  number  of  strokes  per  minute,  the  greater  the 
quantity  of  water  delivered  per  minute.  The  series-wound  motor 
can  be  arranged  to  run  at  varying  speeds,  according  to  the  rate  at 
which  it  has  to  deliver  the  water,  by  either  shunting  its  field  coils,  or 
varying  the  pressure ;  but  a  shunt-wound  motor  with  a  variable 
resistance  in  the  field  coils,  and  with  a  series  coil  arranged  to  assist 
the  shunt  field  coils  at  starting,  is  a  preferable  method.  The  shunt- 
wound  motor,  however,  will  work  better  if  it  is  made  larger 'than 
would  be  absolutely  necessary.  An  increased  torque  will  be  obtained, 
and  the  repairs  bill  will  be  lessened.  Plate  23B  shows  a  variable 
speed  three  cylinder  ram  pump,  driven  by  an  electric  motor. 


The  Bucket  Pump 

So  far  as  the  author  is  aware,  the  bucket  pump  has  not  yet  been 
driven  electrically,  but  there  is  no  reason  that  it  should  not  be,  and 
there  will  be  cases,  where  power  is  generated  at  a  central  station  and 
delivered  to  several  mines,  that  it  will  be  convenient  and  economical 
ro  drive  even  the  large  bucket  pumps  that  are  used  in  some  of  the 
deep  mines  by  electricity.  In  the  bucket  pump,  as  the  name  implies, 
the  water  is  raised  by  one  or  more  buckets,  fitted  with  inlet  and  outlet 
valves.  The  buckets  are  fixed  at  the  end  of  long  rods,  and  are  lowered 
into  the  water  that  is  to  be  raised,  the  inlet  valve  at  the  bottom  of 
the  bucket  opening  as  the  bucket  descends,  and  closing  with  the 
weight  of  the  water  above  it  when  the  bucket  commences  to  ascend, 
the  valve  at  the  top  of  the  bucket  then  opening,  and  the  water  being 
forced  into  a  raising  main  in  the  usual  way.  For  pumping  large 
quantities  of  water,  plunger  and  bucket  pumps  are  sometimes  used 
in  combination,  the  two  being  attached  to  one  set  of  pump  rods.  The 
bucket  pump  is  usually  worked  from  a  beam  engine,  the  pump  rods 
being  attached  to  one  end  of  an  iron  beam  pivoted  at  its  centre,  the 
other  end  of  the  beam  being  attached  to  the  connecting  rod  of  the 
steam  cylinder.  In  some  cases  the  engines  driving  the  pumps  are 
compound,  the  two  cylinders  working  two  beams,  to  which  are  attached 
two  sets  of  buckets,  the  two  working  quite  independently.  In  other 
eases  the  engines  are  made  compound,  but  the  two  cylinders  are  con- 
nected to  the  same  beam,  but  with  different  lengths  of  stroke,  to  fit 


PLATE  22c.— Wound  Rotor  of  Two  Phase  Motor,  with 
Rings  for  connecting  to  Starting  Resistance. 


PLATE  22A. — Messrs.  Siemens 
Bros.'  Motor  Switchboard  for 
Mining  Work. 


PLATE  22B. — Back  of  Messrs. 
Siemens'  Motor  Switch  shown 
in  Plate  22A. 


PLATE  22o. — Diamond  Electric  Rotary  Drill. 


[To  face  p.  296. 


DRIVING    MACHINES   BY   ELECTRICITY         297 

the  position  at  which  their  connecting  rods  meet  the  beam.  The  old 
beam  pumping  engine  has  been  a  very  good  servant,  and  a  very 
economical  one.  The  extreme  case,  that  of  the  Cornish  pumping 
engine,  in  which  a  very  large  steam  cylinder  was  employed,  and  in 
which  the  steam  only  entered  under  the  piston,  which  returned  by 
gravity  and  by  the  pressure  of  the  atmosphere  above,  the  steam 
underneath  it  being  condensed,  held  its  own  for  economy  up  till 
within  very  recent  years.  As  the  beam  pumping  engine  is  always 
fixed  close  to  the  boilers,  a  range  of  boilers  being  sometimes  fixed 
specially  to  supply  it,  it  would  be  difficult  to  introduce  any  economy 
with  electric  driving,  because  every  economy  in  steam  generation  that 
can  be  employed  at  the  electricity  generating  station,  can  be  employed 
at  the  pump  station,  and  the  old  beam  pump,  working  day  and 
night,  is  one  of  the  ideal  constant  loads. '  Where  pumping  has  to 
be  done  at  a  distance  from  the  generating  station,  it  may  be  econo- 
mical to  drive  the  old  beam  pump  by  electric  motors,  in  place  of 
keeping  a  battery  of  boilers  and  the  attendants,  at  the  pumping 
station.  The  arrangement  for  effecting  this  is  very  simple.  The 
steam  cylinders  would  be  moved,  the  connecting  rods  on  the  steam 
side  would  be  replaced  by  rods  sufficiently  long  to  reach  from  the 
beam  end  to  a  crankshaft,  that  would  be  placed  in  any  convenient 
position,  but  would  preferably  occupy  the  pit  from  which  the  steam 
cylinders  have  been  removed.  The  crankshaft  would  be  driven 
directly  by  connecting  it  mechanically  to  the  axle  of  an  electric 
motor,  or  it  could  be  driven  by  ropes  or  belts,  as  convenient. 
Arrangements  could  be  made  also  for  varying  the  speed  when  required 
in  the  manner  described. 


Power  required  for  driving  Pumps 

In  the  early  days  of  electric  driving  of  pumps  and  other  apparatus, 
trouble  sometimes  arose  through  the  motor  not  being  sufficiently 
large.  In  other  words,  proper  calculations  had  not  been  made. 
The  power  required  in  the  electric  motor  is  made  up  of  the  following 
quantities : — 

1.  The  power  required  for  lifting  the  quantity  of  water  that  is  to 
be  raised  per  minute  to  the  height  at  which  it  is  to  be  delivered. 
That  is  to  say,  the  weight  of  the  water  that  is  to  be  raised  per  minute 
in  pounds,  multiplied  by  the  height  to  which  it  is  to  be  raised  in  feet, 
the  horse-power  being  found  by  dividing  the  product  of  these  quanti- 
ties by  33,000.  Pure  water  weighs  10  Ibs.  per  gallon.  The  weight 
of  water  impregnated  with  salts,  such  as  are  found  in  nearly  all  pit 
water,  is  slightly  higher  than  this  ;  but  for  practical  purposes,  if  the 
usual  working  margin  is  allowed  after  making  the  calculations, 


298  ELECTRICITY  IN   MINING 

10  Ibs.  per  gallon  will  not  be  found  far  out.  Thus,  if,  say,  100  gallons 
per  minute  are  to  be  raised  through  330  feet,  the  power  required 
will  be — 

100  x  10  X  330       1A  TTT» 
33,000  H'R 

2.  The  power  required  to  overcome  the  friction  of  the  water  in 
the  pipes  through  which  it  is  forced.  Water  and  air,  when  forced 
through  pipes,  ducts,  etc.,  rub  on  the  sides  of  the  pipes  or  the  ducts,  and 
in  rubbing  create  friction,  and  friction  absorbs  power  in  direct  pro- 
portion to  the  extent  of  the  surface  rubbed,  and  to  the  square  of  the 
velocity  at  which  the  water  or  air  is  flowing,  and  to  a  constant 
depending  upon  the  surface  over  which  it  rubs.  Thus,  the  larger  the 
pipe,  and  therefore  the  larger  its  surface,  the  greater  the  friction. 
Also,  the  longer  the  pipe,  the  greater  the  friction.  From  this  it  would 
appear  as  if  a  larger  pipe  created  more  friction  than  a  small  pipe ;  but 
this  leaves  out  the  question  of  the  velocity  of  the  water.  Where  a 
small  pipe  is  employed,  the  water  has  to  be  forced  through  it  at  a 
higher  velocity  than  with  a  larger  pipe,  and  as  the  friction  increases 
as  the  square  of  the  velocity,  while  it  only  increases  directly  as  the 
surface  of  the  pipe,  the  gain  is  on  the  side  of  the  larger  pipe.  It  is 
usual  to  allow  for  the  friction  created  by  water  passing  through  pipes 
by  reckoning  it  as  so  much  head  that  would  have  to  be  overcome,  if 
the  water  were  lifted  vertically.  The  head  or  vertical  height,  equivalent 
to  any  length  of  any  pipe,  with  any  quantity  of  water  passing  through 
it  at  any  velocity,  is  found  by  taking  the  equivalent  column  that 
would  force  the  water  through  the  length  of  pipe  of  the  given  size  at 
the  velocity  named.  Engineering  pocket-books  give  the  equivalent 
heads  for  different  velocities  of  water,  and  for  different  sizes  of  pipe. 
Thus,  for  50  gallons  of  water  per  minute  passing  through  100  feet  of 
clean,  straight  pipe  of  2-inch  bore,  the  loss  of  head  is  10 '4  feet,  while 
with  a  3-inch  pipe  it  is  only  1*19  feet.  The  loss  of  head  due  to  the 
friction  of  2000  gallons  per  minute  through  100  feet  of  the  same 
pipe,  of  8-inch  bore,  is  12'3  feet.  From  any  of  the  tables  mentioned, 
the  loss  of  head  can  be  obtained  for  any  given  quantity  of  water  per 
minute  passing  through  any  given  length  of  pipe  of  any  given  size ; 
and,  conversely,  the  size  of  pipe  that  will  allow  of  the  passage  of  the 
water  with  only  a  given  loss  of  head  may  be  determined.  Having 
obtained  the  loss  of  head,  the  formula  given  above  is  again  employed, 
and  the  power  required  is  measured  by  the  product  of  the  number 
of  gallons  per  minute  multiplied  by  10,  multiplied  by  the  loss  of 
head  in  feet,  and  divided  by  33,000. 

In  estimating  the  loss  due  to  friction,  however,  it  should  not  be 
forgotten  that  pit  water  usually  contains  salts  in  solution,  which  are 
deposited  upon  the  inside  of  the  pipe,  gradually  lessening  the  bore, 


DRIVING   MACHINES   BY  ELECTRICITY         299 

increasing  the  velocity  of  the  water  if  the  same  quantity  is  to  be 
pumped,  and  increasing  the  loss  of  head  and  the  power  absorbed.  It 
will  be  wise,  therefore,  when  arranging  for  an  electric  motor  to  drive 
a  pump  delivering  water,  as  is  so  frequently  necessary  in  mines, 
through  a  long  line  of  pipe,  to  allow  for  a  loss  of  head  in  a  size  of  pipe 
a  certain  percentage  less  than  that  which  is  actually  fixed.  The 
motor  will  possibly  work  a  little  less  efficiently,  but  the  loss  in  coal 
at  the  generating  station  will  be  more  than  made  up  by  the 
lessened  repairs  bill,  and  by  not  having  the  inconvenience  of  the 
pump  breaking  down  periodically,  when  large  quantities  of  water 
have  to  be  dealt  with,  and  of  having  to  fix  a  larger  motor  after  a 
certain  period. 

Having  obtained  the  power  required  for  (1)  and  (2),  the  efficiency 
of  the  pump  must  next  be  taken  into  consideration.  As  explained, 
the  efficiency  of  the  modern  centrifugal  pump  is  claimed  to  be  70  per 
cent.,  and  in  some  forms  made  by  the  Worthington  Co.  it  is  claimed 
to  be  as  high  as  86  per  cent.  The  efficiency  of  the  slow-moving  ram 
pump  is  not  higher  than  66|  per  cent.,  but  that  of  the  modern  "  Ex- 
press "  pump,  running  at  the  higher  speeds  mentioned,  is  claimed  to 
be  as  high  at  90  per  cent.  The  efficiency  of  the  gearing,  whatever  it 
may  be,  has  also  to  be  taken  into  account,  and  then  the  efficiency  of 
the  motor  itself,  and  the  three  efficiencies  may  be  multiplied  together. 
Thus,  if  we  take  the  efficiency  of  the  pump  at  70  per  cent.,  that 
of  the  gearing  at  95  per  cent.,  and  that  of  the  motor  at  85  per 
cent.,  multiplying  the  three  together  will  give  us  56'5,  and  we 
obtain  the  total  power  that  must  be  delivered  to  the  electric  motor 

by  multiplying  the  sum  of  the  powers  required  by  ^7^-     Thus,  if 

oo'o 

the  power  required  for  the  lift  of  the  water  through  the  vertical 
height  is  10  H.P.,  and  the  power  absorbed  by  the  friction  of  the  pipes 
is  5  H.P.,  making  a  total  of  15  H.P.,  the  power  delivered  at  the 
terminals  of  the  motor,  with  the  above  efficiencies,  must  be 
26*5  H.P.  It  is  also  well  to  remember  another  point.  In  the  above 
calculation  the  efficiency  of  the  motor  was  taken  as  85  per  cent., 
which  will  be  correct  for  any  well-made  modern  electric  motor  when 
it  is  new.  But  if  the  pumping  plant  is  to  be  of  any  service,  it  will 
probably  have  to  work  for  a  number  of  years,  and  the  efficiency  of 
all  parts  will  decline.  The  gearing  will  probably  wear ;  the  pump 
valves,  where  there  are  any,  will  wear,  and  will  allow  slip  of  the 
water  past  them ;  the  efficiency  of  the  electric  motor  will  also  de- 
crease from  various  causes,  particularly  if  it  is  of  the  continuous 
current  type,  and  its  commutator  has  to  be  renewed.  It  is  therefore 
wise  to  consider  the  efficiencies  as  rather  lower  than  those  given  above, 
or,  in  other  words,  to  allow  a  larger  motor.  This  is  a  wise  rule  to 
adopt  in  every  case  where  motors  are  employed  in  mining  work.  It 


300 


ELECTRICITY  IN   MINING 


means  probably  a  little  extra  current  when  the  motor  is  first  run, 
but  it  means  continuous  running,  which  is  of  far  more  importance, 
and  which  saves  the  additional  coal  many  times  over.  It  will  be 
understood  that  the  power  mentioned,  the  26*5  H.P.  or  more  that 
is  to  be  delivered  to  the  motor,  is  the  electrical  energy  actually 
present  in  the  motor,  when  it  is  working,  as  measured  by  the  current 
passing  through  it,  multiplied  by  the  pressure  across  its  terminals. 
Thus,  for  26*5  H.P.,  if  the  pressure  at  the  terminals  of  the  motor 
when  it  is  running  and  furnishing  its  26*5  H.P.  is  500  volts,  the 
current  it  will  be  absorbing  will  be  39*7  amperes,  and  that  is  the 
current  that  must  be  provided  for  the  motor  when  performing  its 
full  work. 


Haulage 

The  next  use  of  electricity  in  mines  was  for  haulage.  There  are 
three  forms  of  haulage,  to  all  of  which  electric  driving  has  been 
applied — endless  rope,  main  and  tail,  and  single  drum.  The  endless 
rope  problem  is  by  far  the  simplest.  The  haulage  system  consists  of 
a  single  rope  coiled  round  a  friction  pulley  at  the  driving  station, 


PREPARED    FOR 
PULLEY    OR    WHEEL 


FIG.  135.— Section  of  David  Bridge's  Friction  Clutch. 

carried  forwards  on  pulleys  between  one  pair  of  rails  to  the  end  of 
the  road,  round  a  tightening  pulley  at  that  end,  and  back  over  rollers 
to  the  driving  station.  One  half  of  the  rope  is  travelling  towards  the 
driving  station,  and  to  this  half  the  full  trams  are  attached  by  clips, 
chains,  and  other  devices.  The  other  half  is  travelling  towards  the 
coal  face,  and  to  this  the  empty  trams  arriving  from  the  surface  are 
also  hitched.  There  is  practically,  when  the  mine  is  working  normally, 
a  uniform  load  upon  the  driving  shaft  of  the  friction  pulley,  and  all 


DRIVING   MACHINES   BY   ELECTRICITY         301 

that  is  required  from  any  form  of  motor  is  rotary  motion  communicated 


FIG.  136.— Coil  Clutch. 


FIG.  137.— Friction  Clutches  for  two  or  more  Sets  of  Endless  Rope  Haulage 
driven  from  the  same  Shaft. 

to  the  driving  shaft.    In  large  collieries  there  are  often  several  endless 
ropes  running  out  to  different  districts  of  the  mine  from  one  driving 


302  ELECTRICITY   IN   MINING 

shaft,  and  the  friction  pulley  belonging  to  each  rope  is  provided  with 
a  friction  clutch,  which  connects  it  to  the  driving  shaft,  and  disconnects 
it  at  will.  Shunt-wound  continuous  and  three-phase  motors  are  the 
most  suitable  for  this  class  of  work,  but  the  same  difficulty  arises 
about  reducing  the  speed.  This,  however,  has  been  easily  overcome 
by  the  interposition  of  second  motion  shafts,  between  the  shaft  of  the 
motor  and  the  driving  shaft  of  the  haulage.  The  question  of  starting 
against  a  heavy  load  does  not  often  arise  here,  as  the  whole  of  the 
haulage  ropes  can  be  disconnected  when  starting  up  by  means  of 
their  friction  clutches.  Figs.  135  and  136  show  forms  of  friction 
clutches,  and  Fig.  137  the  arrangement  of  two  or  more  on  one  shaft. 
Plate  27 A  shows  an  electrically  driven  endless  haulage  plant. 


Power  required  for  Endless  Rope  Haulage 

Where  electric  driving  is  being  introduced  to  take  the  place  of 
driving  by  steam,  compressed  air,  or  ropes,  the  simplest  method  of 
finding  the  power  required  is  to  indicate  the  engines  that  are  doing 
the  work  by  the  method  that  is  being  displaced.  But  where  the 
haulage  is  being  laid  down  new,  or  where  there  is  no  opportunity  of 
obtaining  an  accurate  measurement  of  the  power  being  taken,  the 
calculation  is  a  very  simple  one  to  find  out  what  power  the  motor 
should  be  capable  of  exerting.  The  author  would  again  warn  those 
who  have  matters  of  the  kind  in  hand,  against  the  common  failing  of 
allowing  too  little  power.  It  is  too  often  supposed,  when  electricity 
comes  on  the  ground,  that  it  will  do  the  work  with  a  less  expenditure 
of  energy  than  other  power.  Where  electricity  has  the  advantage  in 
mining  work  is  in  the  smaller  losses  in  transmitting  the  energy. 
Steam,  for  instance,  which  is  now  almost  obsolete,  but  which  was 
employed  very  largely  thirty  years  ago  for  transmitting  power  in 
mines,  is  subject  to  very  heavy  losses  from  condensation  in  the  steam 
pipes.  A  large  portion  of  the  steam  which  should  perform  work  in 
the  engine  driving  the  haulage  or  other  apparatus  is  converted  into 
water  on  its  way  to  the  engine  it  is  to  drive,  and  is  then  not  only 
useless  for  driving,  but  is  often  a  danger  to  the  cylinders  of  the 
engines,  and  to  portions  of  the  steam  pipes.  Compressed  air  is  also 
subject  to  very  heavy  losses,  principally  owing  to  the  leakage  of  the 
air  which  takes  place,  from  the  pipes  which  are  transmitting  it  to  the 
engines  it  is  to  work.  As  mining  engineers  know,  to  their  cost, 
the  floors  of  a  mine  are  constantly  working,  constantly  changing 
their  form,  and  bringing  excessive  strains  upon  the  joints  of  pipes 
which  lie  on  the  floors,  with  the  result  that  leaks  are  frequent,  and 
the  air  delivered  to  the  engine  in-bye  is  very  much  less  than  that 
which  was  compressed  for  the  purpose  on  the  surface.  To  calculate 


DRIVING   MACHINES   BY   ELECTRICITY         303 

the  power  required  for  an  endless  rope  haulage  system,  it  is  first 
necessary  to  find  the  total  weight  of  the  largest  number  of  trams 
which  are  on  the  road  at  any  instant.  As  was  explained,  there 
should  be  the  same  number  of  empty  trams  as  full  trams  on  the 
two  sections  of  the  rope  at  any  moment,  so  that  the  total  weight 
upon  the  rope  is  found  by  taking  the  total  number  of  trams,  or  twice 
the  number  of  either  full  or  empty  trams,  and  adding  to  it  the  total 
weight  of  coal  carried  by  the  trams,  or  the  weight  of  coal  carried  by 
any  individual  tram,  multiplied  by  the  number  of  full  trams  on  the 
road.  Thus,  if  there  be  twenty  empty  trams  and  twenty  full  trams 
upon  the  road,  and  each  full  tram  carries  one  ton  of  coal,  while  each 
tram  itself  weighs,  say,  half  a  ton,  the  total  weight  upon  the  rope  is 
40  x  £  ton  =  20  tons  +  20  X  1  ton  =  also  20  tons,  or  a  total  of  40 
tons.  The  work  required  to  be  performed  in  transporting  the  20  tons 
of  coal  and  the  40  trams  is  the  work  employed  in  overcoming  the 
friction  of  the  tram  wheels  against  the  rails  and  the  tram  axles  them- 
selves in  their  bearings,  or,  where  the  wheels  are  loose,  the  axles  in 
the  hubs  of  the  tram  wheels.  The  frictional  charge,  as  it  is  called, 
has  been  measured  for  a  number  of  trams  working  under  different 
conditions,  and  by  the  latest  determination  it  has  been  found  to  be 
from  32  to  80  Ibs.  per  ton,  according  to  the  condition  of  the  road, 
the  trams,  etc.  That  is  to  say,  when  a  tram  or  a  number  of  trams 
weighing  the  40  tons  mentioned  above  are  being  transported  along 
the  line  of  rails  in  a  mine,  the  work  required  to  transport  them  on 
a  level  road  may  be  as  much  as  40  X  80  Ibs.  =  3200  Ibs.  The  work 
involved  in  transportation  is  measured  by  this  frictional  charge, 
3200  Ibs.,  multiplied  by  the  rate  at  which  the  load  is  being  moved, 
the  number  of  feet  it  is  moved  over  per  minute,  the  work  performed 
in  transporting  when  measured  in  this  way,  being  equivalent  to  the 
work  that  would  have  to  be  performed  in  lifting  the  same  number  of 
pounds  the  same  number  of  feet  vertically  as  it  is  transported  over  in 
a  minute  on  the  level.  Endless  rope  haulage  runs  at  from  1^  to  3 
miles  per  hour,  2  miles  an  hour  being  a  fairly  average  rate,  and 
this  is  equivalent  to  176  feet  per  minute.  So  that  the  work  required 
to  be  performed  in  transporting  the  40  tons  of  mineral  and  waggons 
at  the  rate  of  2  miles  an  hour  will  be  measured  by — 

3200  x  176      17,7Hp 
33,000        :1767H.P. 

In  addition  to  this,  if  the  road  inclines  at  all,  either  against  the  load 
or  with  the  load,  the  circumstance  must  be  taken  into  account  in  the 
calculation.  Thus,  suppose  that  the  road  dips  gradually  1  in  40 
towards  the  face.  This  means  that  the  full  trams  have  to  be  lifted 
the  vertical  height  corresponding  to  that  gradient  in  their  passage 


304  ELECTRICITY   IN   MINING 

from  their  hooking-on  places  to  the  driving  engine.  On  the  other 
hand,  the  empty  trams  have  the  benefit  of  the  falling  gradient,  and 
it  may  be  taken  that  the  weight  of  the  empty  trams  descending 
balances  the  weight  of  the  full  trams,  but  without  the  coal,  ascending 
so  that  the  lifting  of  the  weight  of  coal  through  the  vertical  height 
has  only  to  be  provided  for.  The  question  arises  here  also  as  to  the 
average  height  through  which  the  coal  has  to  be  lifted.  The  filled 
trams  are  hooked  on  at  various  points  along  the  road,  where  branch 
roads,  or,  as  they  are  called,  secondary  haulage  roads  lead  to  the  face 
of  the  coal.  If  we  assume  the  total  length  of  the  haulage  road  to 
be  one  mile,  and  that  the  trams  commence  to  hook  on  at  half  a 
mile,  so  that  those  from  the  nearest  station  have  to  be  transported 
through  half  a  mile  to  the  hauling  engine,  and  through  the  vertical 
height  due  to  half  a  mile,  while  the  tram  at  the  farthest  hooking-on 
place  has  to  be  transported  a  mile  and  lifted  through  the  vertical 
height  corresponding  to  a  mile,  we  shall  not  be  far  wrong  if  we 
take  three-quarters  of  a  mile  as  the  distance  and  the  height  due 
to  that  as  the  vertical  lift.  In  addition  to  the  above  there  is  the 
power  required  for  moving  the  rope  itself,  which  varies,  of  course, 
according  to  the  weight  of  the  rope,  the  number  of  rollers,  sheaves, 
etc.,  it  travels  over,  and  the  rate  at  which  it  travels. 

Having  obtained  the  power  required  to  overcome  the  friction  of 
the  mine  wagons,  that  required  for  the  vertical  lift,  if  any,  and  that 
required  for  moving  the  rope,  we  have  in  the  sum  of  these  the  total 
power  that  must  be  delivered  to  the  rope  itself.  To  this  quantity 
must  be  added  the  power  absorbed  by  the  friction  pulleys,  axles,  etc., 
this  total  making  the  power  that  must  be  delivered  to  the  driving 
axle  of  the  friction  pulleys.  As  in  the  case  of  pump  driving,  we  have 
then  to  add  the  power  absorbed  by  the  second  motion  shaft,  where 
one  is  employed,  as  is  most  usual,  and  by  the  motor  itself.  Taking, 
as  before,  the  efficiency  of  the  gearing  as  95  per  cent.,  and  that  of  the 
motor  as  90  per  cent.,  the  combined  efficiency  of  the  two  will  be 
85J,  and  the  total  power  arrived  at,  as  described  above,  must  be 

multiplied  by  -—  .     The   author   would   give  the  same  caution  in 


this  case  as  in  the  case  of  pump  driving,  and  would  strongly  advise 
that  in  the  calculations  the  efficiency  of  the  motor  and  the  gearing 
should  not  be  taken  at  their  best,  because,  as  in  other  cases,  the 
efficiency  decrease  with  time  and  wear,  and  it  will  be  safer  if  the 
motor  is  taken  at  80  per  cent,  and  the  gearing  at  90  per  cent.,  the 
combined  efficiency  being  taken  at  72  per  cent. 

The  number  of  trams  that  will  be  upon  the  road  when  the  rope  is 
fully  loaded  will  be  found  by  taking  the  output  that  is  required  from 
the  particular  district,  and  the  quantity  of  coal  carried  by  each  tram. 

The  single  drum  or  dip  haulage  motor  also  presents  a  simple 


PLATE  23A. — Single  Chamber,  High  Lift  Centrifugal  Pump,  driven  by  Electric 
Motor.     Messrs.  Mather  &  Platt. 


PLATE  23B. — Mather  &  Platt's  Variable  Stroke  Three  Throw  Ram  Pump, 
driven  by  an  Electric  Motor.  The  three  Cylinders  are  arranged  round 
the  Containing  Cylinder,  and  the  Stroke  is  regulated  by  the  Lever  on 
the  Eight. 

[To  face  p.  304. 


DRIVING  MACHINES   BY  ELECTRICITY         305 

problem.  It  consists  usually  of  a  drum  with  a  rope  coiled  on  it,  the 
rope  being  driven  by  any  convenient  source  of  power.  The  rope  is 
usually  carried  down  a  dip  to  the  face,  where  it  is  attached  to  a  full 
tram,  which  is  hauled  up  to  the  top  of  the  dip  or  "  brow  "  by  winding 
the  rope  up  on  the  drum.  Series,  shunt-wound,  and  three-phase 
motors  are  applicable  to  this  work,  but,  preferably,  the  shunt-wound 
motor,  with  a  series  coil  added  to  give  starting  torque.  It  is  some- 
times arranged  for  one  end  of  the  rope  to  lower  one  or  more  tubs 
down,  while  the  other  end  is  hauling  one  or  more  up,  the  motor,  if 
one  is  employed,  having  only  to  furnish  the  difference  between  the 
energy  given  out  by  the  descending  tubs  and  that  taken  by  the 
ascending  tubs. 


Power  required  for  Single  Drum  Haulage 

Single  drum  haulage  plants  may  be  used  for  short  "  staple  "  pits, 
from  one  seam  to  another  within  the  mine,  also  for  winze  hoists  in 
metalliferous  mines,  and  for  hauling  single  or  a  small  number  of  trams 
up  dip  roads,  leading  from  the  face  to  the  main  haulage  road.  The 
calculation  for  the  two  is  not  quite  the  same.  For  the  simple  hoist 
or  wind  the  power  required  is  measured  by  the  weight  of  the  cage 
and  its  load,  or  the  skip  and  its  load,  plus  the  weight  of  the  hoisting 
rope,  multiplied  into  the  vertical  height  lifted.  This  is  the  total 
power  required  in  foot-pounds,  and  the  horse-power  required  is 
found  by  taking  the  lift  in  feet  in  one  minute,  multiplying  it  into  the 
total  weight,  as  described  above,  and  dividing  by  33,000 ;  or,  where  the 
time  is  very  short,  taking  the  total  lift  in  one  second,  multiplying  by  the 
weight,  and  dividing  by  550.  This  gives  the  horse-power  that  must  be 
delivered  to  the  rope  that  is  to  raise  the  cage  or  skip  with  its  load. 

To  this  power  must  be  added  the  power  absorbed  in  friction  by 
the  drum  upon  which  the  winding  rope  is  wound  up,  that  absorbed 
by  the  gearing,  if  any,  and  that  absorbed  by  the  motor.  Or  taking 
again  the  efficiency  of  the  motor  as  90  per  cent.,  and  the  combined 
efficiency  of  the  gearing  and  drum  as  80  per  cent.,  the  power  to  be 
delivered  to  the  rope  must  be  multiplied  by  y^9.  Where  the  single 
drum  is  employed  to  pull  up  an  incline,  the  power  required  is  made 
up  of  two  quantities — that  required  to  raise  the  load,  consisting  of  the 
tram  and  its  load  of  mineral  through  the  vertical  height,  and  that 
required  to  overcome  the  friction  of  the  tram  wheels  upon  the  rails 
and  upon  their  axles.  These  are  found  in  the  same  way  as  described 
with  endless  haulage,  and  the  total  power  is  equated  with  the 
efficiencies  of  the  haulage  drum  and  the  motor — say  a  combined 
efficiency  of  72  per  cent.,  as  described  above.  Plate  27fi  shows  an 
electrically  driven  single  drum  haulage  plant. 


306  ELECTRICITY  IN   MINING 


Main  and  Tail  Haulage 

The  main  and  tail  haulage  is  the  most  difficult  of  all  to  deal  with. 
With  this  arrangement  there  is  practically  an  endless  rope,  but  it 
consists  of  two  ropes,  called  respectively  the  main  and  the  tail  rope. 
The  main  rope  is  attached  to  the  front  of  a  journey  of  trams,  the  tail 
rope  being  attached  to  the  rear  tram,  the  trams  completing  the 
loop.  The  trams,  as  they  are  filled,  are  hauled  to  a  certain  point,  to 
which  the  main  and  tail  ropes  are  also  brought.  They  are  pulled  out 
to  the  haulage  station  by  winding  the  main  rope  on  its  drum.  The 
tail  rope  at  the  same  time  being  run  out  off  its  drum,  it  being  taken 
round  a  pulley  in  the  same  position  as  the  tightening  pulley  of  the 
endless  haulage,  and  attached  to  the  rear  tram.  When  the  full  trams 
have  been  pulled  out  to  the  haulage  station,  and  passed  on  to  the  pit 
bottom  or  to  the  main  haulage,  as  is  frequently  arranged,  a  journey 
of  empty  trams  is  made  up  which  complete  the  loop  between  the 
main  and  tail  ropes,  and  they  are  pulled  out  to  the  station  from  which 
the  full  trams  were  brought  by  winding  up  the  tail  rope  on  its  drum, 
and  allowing  the  main  rope  to  run  out  behind  the  trams.  The  problem 
is  more  difficult  than  the  endless  rope  problem,  because  the  motor 
has  to  start  against  the  full  load,  and  because  the  haulage  roads  are 
nearly  always  very  irregular.  During  one  portion  of  the  run  out  the 
load  may  be  running  up  an  incline,  and  during  another  portion  it  may 
be  running  down  an  incline,  while  during  a  third  it  may  be  on  the 
level. 

Further,  as  the  load  approaches  the  haulage  station,  the  speed  has 
to  be  lessened  gradually,  and  the  journey  brought  quietly  to  rest,  in 
such  a  manner  that  the  horses,  which  are  often  employed  to  draw  it 
to  the  pit  bottom,  or  to  the  main  haulage,  can  easily  handle  it.  The 
shunt-wound  motor  and  the  three-phase  motor  are  again  the  most 
suitable,  the  shunt-wound  motor  having  a  series  coil  added  for  start- 
ing torque,  and  its  speed  being  controlled  by  varying  the  current  in 
the  field  circuit.  The  speed  of  the  three-phase  motor  is  controlled  by 
inserting  resistance  in  the  rotor  circuit,  which  is  brought  into  opera- 
tion only  when  speed  is  to  be  lessened,  and  when  the  journey 
is  to  be  brought  to  rest.  At  the  recent  Colliery  Exhibition, 
the  Lahmeyer  Co.  showed  what,  in  the  author's  view,  was  a 
very  well  worked  out  arrangement.  The  three-phase  rotor  was 
wound  for  only  two  phases,  the  stator  being  wound  for  three. 
Resistances  of  sufficient  size  were  arranged  to  be  thrown  into  the  rotor 
circuits,  and  the  whole  was  controlled  by  a  horizontal  drum,  controlled 
on  the  lines  of  the  tramcar  controller.  The  controller  itself,  and  a 
reversing  switch,  were  operated  at  a  convenient  distance  by  a  long 
lever,  very  similar  to  the  ordinary  steam  engine  lever,  which  was 


DRIVING  MACHINES   BY   ELECTRICITY         307 

thrown  forward  or  backward  to  insert  or  take  out  resistance,  and  to 
right  or  left,  to  direct  the  current  normally  or  reverse,  so  that  the 
attendant  had  the  apparatus  completely  under  control,  and  with  a 
very  simple  arrangement.  Plate  27c  shows  an  electrically  driven 
main  and  tail  haulage  plant. 


Power  required  for  Main  and  Tail  Haulage 

The  motor  required  with  main  and  tail  haulage  is  always  larger 
than  that  required  with  endless  rope  haulage,  because  a  larger 
quantity  of  mineral  has  to  be  drawn  out  at  one  journey,  and  at  a 
higher  speed.  While  the  load  in  the  case  of  the  endless  rope  haulage 
system  is  uniformly  distributed  throughout  the  rope,  and  the  rope  is 
always  working,  always  receiving  a  load  both  for  the  pit  bottom  and 
for  the  face,  and  is  always  delivering  coal  at  the  pit  bottom  and 
delivering  waggons  at  the  face,  with  main  and  tail  haulage  the  work 
is  done  more  or  less  spasmodically.  As  explained,  a  journey  of 
trams  making  up  a  considerable  quantity  of  coal,  is  made  up  at 
intervals,  and  is  pulled  rapidly  out  to  the  pit  bottom.  While  the 
endless  rope  also  runs  at  only  an  average  of  two  miles  an  hour,  the 
main  and  tail  ropes  run  usually  at  from  six  to  ten  miles  an  hour. 
Hence  the  greater  economy  of  the  endless  rope  system  in  engine 
power.  While  the  actual  work  done  is  the  same,  the  same  quantity 
of  coal  being  drawn  over  the  same  distance,  with  the  endless  rope  a 
long  time  is  taken  in  the  transportation,  and  so  a  smaller  engine  or 
motor  is  able  to  do  the  work.  The  great  advantage  of  the  main  and 
tail  system  over  the  endless  rope  is  the  fact  that  only  a  single  road 
is  necessary,  while  a  double  road  is  required  with  the  endless  rope, 
and  this  delayed  the  adoption  of  the  endless  rope  system  for  a  very 
long  period.  With  main  and  tail  haulage  the  power  required  is 
made  up,  as  before,  of  two  portions,  that  required  to  overcome  the 
friction  of  the  trams  on  the  rails,  etc.,  and  that  required  to  provide 
the  vertical  lift.  As  before,  the  power  required  to  overcome  the 
friction  is  obtained  by  taking  the  weight  of  the  largest  number  of 
trams  that  may  compose  a  journey,  plus  the  weight  of  the  largest 
quantity  of  mineral  that  the  trams  may  carry,  and  allowing  80  Ibs. 
per  ton  of  this  quantity,  multiplying  the  amount  so  obtained  by  the 
distance  travelled  over  in  one  minute,  880  feet,  where  the  rate  of  trans- 
portation is  ten  miles  an  hour,  and  divided  by  33,000.  The  power 
required  for  the  vertical  lift  is  found  in  the  same  way  as  has  been 
described  in  connection  with  endless  rope  and  single  drum  haulage, 
but,  as  was  mentioned,  main  and  tail  haulage  roads  are  often  very 
irregular,  and  the  power  required  for  the  vertical  lift  will  be  that 
required  to  transport  the  journey  up  the  steepest  rise  between  the 


308  ELECTRICITY  IN   MINING 

hooking-on  place  and  the  hauling  engine,  at  the  rate  at  which  the 
journey  is  travelling.  The  power  required  to  drive  the  rope  must 
also  be  taken  into  consideration,  as  with  endless  rope  haulage,  and 
the  total  amount  equated  with  the  efficiencies  of  the  motor  gearing 
and  drums,  as  already  explained. 


Transmitting  the  Power  from  the  Electric 
Motor  to  the  Haulage  Gear 

In  the  early  days  of  mechanical  haulage,  steam  and  compressed 
air  driven,  there  was  only  one  method  of  transmitting  the  power 
from  the  crankshaft  of  the  engine  to  the  shaft  of  the  haulage  plant, 
viz.  by  means  of  spur  gearing.  Spur  gearing  has  the  advantage  that 
it  is  very  good  natured,  it  will  go  on  working,  provided  that  the  power 
is  delivered  to  it,  when  other  gearing  would  refuse,  but  it  is  apt, 
in  most  mines,  to  be  a  great  waster  of  power.  In  coal  mines  in 
particular,  coal-dust  gets  in  between  the  wheels  and  creates  con- 
siderable friction,  and  the  same  thing  is  apt  to  be  met  with  in 
metalliferous  mines.  Where,  as  in  some  instances,  short  ropes  have 
been  employed  to  displace  a  portion  of  the  gearing,  there  has  been 
less  chance  for  an  increase  of  friction  from  dirt  between  the  wheels  of 
the  gearing,  because  there  were  fewer  wheels,  but  the  rope  drive  has 
itself  introduced  a  loss  of  often  as  much  as  ten  per  cent.  The  only 
other  method  is  by  worm  gearing,  and  up  till  recently  this  was  very 
inefficient.  Late  developments,  however,  combined  with  better  know- 
ledge of  the  subject,  and  better  tools,  have  enabled  special  makers  of 
worm  gearing  to  produce  gear  which  it  is  claimed  has  an  efficiency 
as  high  as  eighty-five  per  cent,  and  over.  Though  this  efficiency  is 
not  as  high  as  spur  gearing  when  new,  it  will  probably  remain  at 
or  about  its  initial  efficiency,  with  reasonable  care,  long  after  spur 
gearing  has  been  reduced  considerably  below  that  figure.  Apart 
from  the  question  of  efficiency,  worm  gearing  is  the  ideal  arrangement 
for  power  transmission  for  haulage  gear.  It  enables  the  haulage 
motor  to  be  fixed  on  an  extension  of  the  bedplate  carrying  the 
haulage  drums  or  friction  pulleys,  and  the  worm  gearing  enclosed  in 
an  oil  chamber  to  be  fixed  well  out  of  the  way  on  the  same  bedplate, 
and  so  as  to  transmit  the  power  evenly  and  continuously  to  the  shaft 
of  the  haulage  gear.  It  must  be  remembered,  however,  that  when 
worm  gear  is  employed,  the  additional  power  required  must  be 
provided  in  the  motor.  And,  as  was  explained  in  connection  with 
the  driving  of  pumps,  the  power  the  calculation  shows  that  is  required 
to  be  delivered  to  the  motor,  must  be  that  found  by  taking  the  current 
passing  through  the  motor  when  doing  its  full  work,  multiplied  by 
the  pressure  of  the  service  across  its  terminals  at  the  same  instant. 


DRIVING   MACHINES   BY    ELECTRICITY         309 


Overhead  Rope  Railways 

The  overhead  rope  railway  is  a  very  useful  apparatus  for  trans- 
porting coals  or  rubbish  across  a  valley,  especially  where  either  a 
railway  or  a  river,  or,  as  often  happens,  both  run  in  the  bottom  of 
the  valley,  or  in  certain  cases  for  transporting  them  either  up  or 
down  a  steep  hillside,  and  for  other  conditions.  There  are  different 
forms  of  rope  railways,  but  they  are  all  on  certain  lines.  There  is 
always  a  stout  wire  rope  stretched  across  the  valley  or  space  to  be 
spanned,  maintained  as  tight  as  conditions  will  allow,  and  the  carriage 
or  truck  carrying  the  coal  or  the  rubbish  is  suspended  under  the  rope 
by  hangers  depending  from  two  substantial  double-flanged  wheels 
which  run  on  the  rope.  A  haulage  rope,  which  may  be  endless  or 
single,  practically  completes  the  apparatus.  Where  one  side  of  the 
span  is  higher  than  the  other,  the  load  is  allowed  to  descend  by 
gravity,  and  is  pulled  up  by  the  haulage  rope,  which  is  worked  by 
a  small  engine  and  haulage  drum  at  the  top.  Where  the  two  stations 
are  approximately  level,  there  are  sometimes  two  engines,  with  two 
haulage  ropes,  one  at  each  end,  one  pulling  the  load  in  one  direction, 
and  the  other  pulling  it  in  the  opposite  direction,  and  sometimes  the 
rope  is  worked  from  one  end  and  is  endless,  the  direction  of  motion 
of  the  engine  being  reversed  when  the  direction  of  transportation  of 
the  load  is  reversed.  The  haulage  drum  in  this  case  may  conveniently 
be  worked  by  an  electric  motor,  where  there  is  one  on  the  ground, 
and  it  will  preferably  be  of  the  shunt-wound  type,  and  may  be 
geared  to  the  haulage  drum  by  spur  or  worm  gearing,  as  convenient. 
The  power  required  will  be  found  in  a  similar  manner  to  that 
described  for  ordinary  haulage.  There  will  be  the  vertical  lift,  where 
one  station  is  higher  than  the  other,  and  there  will  be  the  friction  of 
the  flanged  wheels  on  the  rope. 

A  modification  of  this,  which  was  introduced  some  time  ago  by 
Messrs.  Brothers,  consists  of  what  is  practically  an  electric  locomotive 
running  on  the  stretched  cable  by  means  of  the  two  flanged  wheels, 
but  taking  current  from  a  copper  wire  stretched  between  the  stations, 
connection  being  made  by  means  of  another  pulley,  arranged  for  the 
purpose  running  on  the  wire. 

There  is  also  the  arrangement  known  as  the  Telpher  system. 


Winding  by  Electricity 

Winding  is  the  crux  of  the  whole  problem  of  working  mines,  and 
particularly  coal  mines,  by  electricity.  In  coal  mines  the  winding 
engine  absorbs  about  fifty  per  cent,  of  the  total  power  generated  at 


310  ELECTRICITY   IN   MINING 

the  colliery,  so  that  if  winding  can  be  done  economically  electricity 
has  a  very  good  chance  of  success,  and  vice  versa.  The  winding 
problem  is  itself  a  somewhat  difficult  one.  The  usual  arrangement 
is,  there  are  two  cages,  one  of  which  will  be  at  the  bottom  of  the 
shaft  with  its  load  of  full  trams,  the  other  at  the  top  of  the  shaft 
with  the  empty  trams.  When  the  wind  is  started,  sufficient  energy 
must  be  present  to  move  the  load  at  the  shaft  bottom  from  rest,  to 
lift  the  weight  of  the  wire  rope  attached  to  the  cage,  to  overcome  the 
inertia  of  the  winding  drum  and  the  winding  engine  itself,  and  to 
furnish  the  power  required  for  acceleration.  As  the  wind  proceeds 
the  weight  of  the  ascending  rope  becomes  less,  while  that  of  the 
descending  rope  becomes  greater,  and  the  descending  cage  is  acquir- 
ing momentum  every  instant,  till  at  a  certain  period  of  the  wind 
the  momentum  of  the  descending  cage  is  sufficient  to  perform  the 
remainder  of  the  work  involved  in  raising  the  ascending  cage  to 
the  bank.  Any  one  who  watches  a  steam  winding  engine  at  work, 
particularly  if  it  exhausts  into  the  atmosphere,  will  notice  that 
considerable  power  is  exerted  when  the  wind  commences,  that  it 
gradually  decreases,  and  some  sensible  time  before  the  ascending 
cage  arrives  at  the  bank,  steam  has  been  completely  cut  off  from  the 
engine.  Part  of  this  difficulty  has  been  overcome  by  what  is  known 
as  the  "  Koepe "  system,  in  which  a  balance  rope  is  employed.  A 
wire  rope  is  attached  to  the  under  side  of  both  cages,  the  loop  passing 
under  a  pulley  in  the  sump  at  the  bottom  of  the  shaft.  The  winding 
is  performed  also  by  a  single  rope,  the  ends  of  which  are  attached  to 
the  upper  sides  of  the  cages,  the  driving  being  performed  by  a  friction 
pulley,  or  similar  arrangement,  so  that  the  engine  has  only  to  over- 
come the  friction  of  the  whole  apparatus,  and  to  raise  the  net  load 
of  the  mineral. 


The  Sources  of  Waste  in  Winding 

One  great  source  of  waste  in  connection  with  winding  engines  is 
condensation  of  steam.  As  explained  above,  steam  is  shut  off  in 
the  great  majority  of  winding  engines  several  strokes  before  the 
completion  of  the  wind,  and  the  engine  stands  without  steam  in  its 
cylinder  while  the  cages  are  being  unloaded  and  reloaded.  Further, 
the  raising  of  mineral  usually  only  occupies  from  eight  to  ten  hours 
of  the  day,  while  the  winding  engine  must  be  ready  to  raise  or  lower 
men,  timber,  tools,  horses,  etc.,  at  any  time  during  the  remainder  of 
the  twenty-four  hours.  During  the  period  the  engine  is  standing 
condensation  goes  on  very  rapidly. 

Though  a  very  large  effort  has  to  be  exerted  by  the  steam  engine 
when  the  wind  commences,  that  is  to  say,  when  the  cage  is  first 


DRIVING   MACHINES   BY    ELECTRICITY         311 

lifted  from  the  bottom,  that  is  not  the  period  at  which  the  greatest 
expenditure  of  power  takes  place.  It  is  after  this,  when  the  engine 
has  taken  the  weight  of  the  cage  and  the  rope,  and  has  commenced 
to  draw  it  up  the  shaft,  and  is  rapidly  increasing  the  rate  at  which 
it  is  raising  it.  The  problem  is  very  similar  to  that  of  the  locomotive 
on  a  railway,  when  starting  from  rest.  Great  power  is  required  to 
get  up  speed,  or,  as  it  is  termed,  to  provide  for  the  acceleration,  the 
increase  of  speed  up  to  the  running  rate,  a  certain  increase  taking 
place  in  each  instant.  Thus  it  is  during  the  period  of  the  wind, 
after  the  cage  is  lifted  and  before  the  descending  cage  has  acquired 


FIG.  138. — Diagram  of  Connections  of  Three-phase  Electric  Winding  Plant  at 
Preussen  II.  Colliery.  A  is  the  Stator,  B  is  the  Rotor,  C  the  Liquid  Controller, 
D  the  Reversing  Switch,  E  the  Controllers,  and  F  the  Winding  Drum. 

much  momentum,  that  the  great  expenditure  of  power  is  required. 
After  the  descending  cage  has  acquired  sufficient  momentum  to 
perform  the  remainder  of  the  wind,  it  is  also  increasing  its  momentum 
every  instant,  while  the  weight  that  is  being  lifted  is  also  decreasing 
with  every  instant.  Hence  there  is  a  surplus  of  power  in  the  latter 
portion  of  the  wind  that  is  unused  in  steam  winding,  and  it  is  the 
endeavour  of  nearly  every  system  of  electrical  winding  to  utilize  this 
hitherto  wasted  energy,  to  assist  with  the  heavy  expenditure  of  energy 
in  the  early  part  of  the  wind,  and  so  to  lessen  the  size  of  the  motor 
employed,  etc. 


3I2 


ELECTRICITY   IN   MINING 


The  earliest  application  of  electric  winding  was  to  staple  pits,  the 
arrangement,  as  already  explained  on  p.  305,  being  merely  a  modi- 
fication of  the  single  drum  dip  haulage,  and  being  worked  either  by 
shunt- wound  continuous  current,  or  three-phase  motors. 

The  shunt-wound   and  series-wound  continuous  current  motors 

and  the  three-phase  motor 
have  been  also  adapted 
in  America  and  on  the 
Continent  for  winding 
from  the  main  shaft,  by 
simply  gearing  the  motor 
to  the  axle  of  the  winding 
drum,  just  as  with  a 
haulage  plant,  the  start- 
_  ing  switch  and  resistance 
S  |  ?  M  5  being  made  to  be  worked 
by  a  lever,  similar  to  the 
lever  used  with  steam 
engines.  It  is  doubtful 
whether,  in  some  cases, 
this  arrangement  is  so 
wasteful  as  it  seems.  The 
complaint  is  made  that 
a  considerable  waste  of 
current  takes  place  in 
the  starting  resistance, 
and  this  is  quite  correct ; 
but  as  with  ordinary 
motors  of  every  kind, 
the  starting  resistance  is 
only  in  circuit  for  a  very 
short  interval,  and  though 
it  is  repeated  at  every 
wind,  it  is  easy  to  con- 
ceive conditions  under 
which  the  loss  from  this 
cause  would  not  be 
serious,  and  would  not 
make  up  for  the  interest 
on  the  very  heavy  cost 
of  more  economical  plant.  There  is  also  the  other  complaint 
which  has  been  alluded  to  above,  that  the  later  portion  of  the 
wind,  when  the  energy  is  being  given  out  by  the  descending 
cage,  is  not  made  use  of  to  assist  the  economical  working  of  the 
apparatus.  With  very  deep  mines,  and  where  very  rapid  winding 


.  .A  ,.;.^: 


if! 


JJ. 


PLATE  24A. — Worthington  Multiple 
Stage,  High  Lift  Centrifugal  Pump, 
driven  by  an  Electric  Motor  and 
arranged  for  Sinking. 


PLATE  24s.— Electrically  Driven  Three  Throw  Earn  Pump, 
arranged  for  Sinking,  by  Messrs.  Frank  Pearn  &  Co. 


[To  face  p.  312. 


DRIVING   MACHINES   BY   ELECTRICITY 


3*3 


is  necessary,  as  in  some  of  the  deep  mines  of  the  United  Kingdom, 
and  in  the  large  gold  mines  on  the  Rand  and  elsewhere,  where  a  very 
large  output  is  absolutely  necessary  to  give  a  return  for  the  very 
heavy  outlay  in  sinking,  etc.,  the  criticism  is  well  founded  ;  but  with 
shallow  mines,  and  with  mines  such  as  some  metalliferous  mines, 
where  winding  is  not  at  high  speed,  the  loss  due  to  the  non-use  of  the 
energy  liberated  by  the  descending  cage  is  not  great.  There  is,  how- 
ever, another  and  a  more  important  objection,  and  that  is  that  when 
the  wind  starts,  a  very  large  current  is  necessary  in  order  to  provide 
the  large  starting  torque,  and  this  may  have  a  serious  effect  upon  the 
pressure  of  the  service  delivered  at  the  mine,  where  the  mine  is 
receiving  current  from  a  generating  station  at  a  distance,  and  upon 
the  generating  station  itself,  if  it  is  designed  to  work  very  close  to  its 


FIG.  140.— Plan  of  Winding  Gear  at  Zollern  II.  Colliery.    Lettera  refer  to  Parts 

as  in  Fig.  139. 

possible  output.  Mr.  W.  C.  Mountain,  of  Messrs.  Ernest  Scott  & 
Mountain,  reports  that  at  Preussen  II.  Colliery  in  Germany,  where 
the  winding  is  done  by  three-phase  motors  directly  connected  to  the 
winding  drums,  when  the  wind  started,  a  drop  of  from  600  to  900 
volts  from  an  initial  pressure  of  2300  to  2400  volts  took  place,  and 
that  it  required  two  engines,  each  of  750  H.P.,  and  two  generators, 
each  of  550  K.W.,  to  furnish  the  necessary  current  for  winding.  The 
pit  in  this  case  is  600  yards  deep,  and  the  winding  speed  was  52  feet 
per  second  when  drawing  coal.  But  again,  with  shallow  pits  and 
with  small  outputs,  and  with  low  speeds,  etc.,  in  fact  anywhere  but 
in  the  case  of  the  large  mines  mentioned,  this  may  not  be  serious. 
In  a  great  many  instances  it  would  not  be ;  and  meanwhile  the 
arrangement  has  the  great  advantage  of  simplicity  and  low  cost.  Plate 


314  ELECTRICITY   IN    MINING 

28A  shows  an  electrical  winding  plant  in  a  German  colliery;  the 
frontispiece,  the  electrical  winding  plant  at  Lens  colliery  in  France ; 
Fig.  138,  a  diagram  of  the  winding  plant  at  Preussen  II.  colliery  ; 
Figs.  139  and  140,  that  at  Zollern  II.  colliery. 


The  Siemens- Ilgner  Winding  Arrangement 

The  Siemens-Ilgner  apparatus  was  the  first  to  seriously  attack 
the  electrical  winding  problem  on  economical  lines,  and  the  idea 
underlying  both  the  early  form  of  the  apparatus  in  which  accumu- 
lators were  employed,  and  the  later  form  in  which  a  flywheel  is 
used,  was  to  absorb  the  energy  given  out  by  the  descending  cage,  and 
to  use  that  energy,  or  all  of  it  that  is  available  after  charges  for 
storage  and  conversion  have  been  met,  to  assist  in  starting  the  cage 
from  rest,  and  in  meeting  the  heavy  charges  for  acceleration  at  the 
commencement  of  the  wind.  In  the  Siemens-Ilgner  apparatus  the 
current  from  the  supply  station  is  brought  to  a  motor  generator 
consisting,  as  explained  in  Chapter  IV.,  of  a  motor  whose  rotating 
member  is  mechanically  connected  to  the  armature  of  a  generator, 
the  motor  receiving  current  from  the  supply  station  at  whatever 
pressure,  and  in  whatever  form  it  is  convenient  to  deliver  it,  a 
stationary  transformer  being  employed  where  necessary  to  transform 
the  pressure  down  to  any  convenient  figure.  On  the  axle  of  the 
motor  generator  is  a  heavy  flywheel,  specially  constructed  to  run 
with  safety  at  a  high  velocity,  and  it  is  in  this  flywheel  that  the 
energy  liberated  by  the  descending  cage  is  stored,  and  from  which  it 
is  delivered,  in  the  well-known  flywheel  manner,  on  the  next  wind. 
A  direct  current  motor  is  geared  to  the  winding  drum  by  spur  gearing 
in  the  usual  way,  two  motors  being  employed,  one  on  each  side  of 
the  winding  drums  in  some  cases.  The  armature  of  the  generator 
of  the  motor  generator  and  that  of  the  winding  motor  are  connected 
in  series,  so  that  when  the  motor  calls  for  current,  as  explained  in  a 
previous  part  of  this  chapter,  it  receives  it  from  the  motor  generator, 
which  in  its  turn  calls  for  it  from  the  generating  station.  The 
pressure  at  which  the  current  of  the  motor  generator  is  generated  is 
regulated  by  an  adjustable  resistance  connected  in  the  field  circuit  of 
the  generator  side  of  the  motor  generator,  the  quantity  of  resistance 
in  circuit  at  any  instant  being  adjusted  by  the  engine  man's  lever. 
The  working  of  the  arrangement  is  as  follows :  When  starting  to 
wind,  a  certain  exciting  current  is  delivered  to  the  generator  of  the 
motor  generator,  and  a  certain  current  passes  from  it  to  the  motor  of 
the  winding  engine,  the  current  being  sufficiently  strong  to  provide 
the  torque  necessary  to  start  the  winding  drum,  rope,  etc.,  from  rest. 
After  the  cage  has  been  lifted  from  the  bottom,  the  excitation  of  the 


DRIVING   MACHINES   BY   ELECTRICITY         315 

motor  generator  is  increased  gradually,  this  providing  the  necessary 
increase  of  current  to  furnish  the  acceleration  required  to  provide  the 
increased  speed  of  the  cage.  As  the  wind  proceeds,  the  excitation  of 
the  motor  generator  is  gradually  lessened  after  the  acceleration  period 
is  passed,  and  after  the  point  is  reached  when  the  energy  being 
delivered  by  the  descending  cage  is  sufficient  to  perform  the  wind, 
the  excitation  of  the  motor  generator  is  made  such  that  its  pressure 
is  less  than  that  of  the  back  pressure  created  by  the  winding  motor. 
As  the  wind  goes  on  and  the  descending  cage  drives  the  drum  and 
the  motor,  the  latter  delivers  current  to  the  motor  generator,  driving 
the  generator  as  a  motor,  the  energy  so  delivered  being  stored  in  the 
flywheel,  and  the  arrangement  enabling  the  motor  to  be  brought 
easily  and  quickly  to  rest  on  arriving  at  bank.  When  the  next  wind 
commences,  in  place  of  the  heavy  starting  current  being  required 
from  the  generating  station,  the  flywheel  of  the  motor  generator 
delivers  up  a  portion  of  the  energy  it  received  in  the  latter  part  of 
the  last  wind,  and  this  enables  the  starting  current  to  be  considerably 
reduced.  The  energy  in  the  flywheel  also  assists  the  motor  during 
the  acceleration  period,  the  result  being  that  the  call  upon  the 
generating  station  is  very  nearly  uniform  throughout  the  mineral 
winding  period.  In  the  first  arrangement  worked  out  by  Messrs. 
Siemens  and  Herr  Ilgner,  an  electrical  accumulator  was  used  in 
place  of  the  flywheel,  the  accumulator  absorbing  the  current  delivered 
to  it  by  the  motor  during  the  later  period  of  the  wind,  and  delivering 
current  to  the  motor  during  the  early  period  of  the  wind,  very  much 
after  the  same  manner  as  the  automatic  reversible  booster  does.  It 
was  found,  however,  that  sparking  at  the  switches  gave  a  great  deal 
of  trouble,  as  the  currents  were  necessarily  very  large,  and  it  was 
abandoned  in  favour  of  the  flywheel.  The  question  is  one  of  flywheel 
versus  accumulator,  or  mechanical  versus  electrical  storage  of  power. 
There  is  a  certain  amount  of  danger  in  a  heavy  flywheel  running 
at  a  high  speed,  and  there  is  considerable  difficulty  in  handling  an 
accumulator  under  the  conditions  at  a  colliery.  Fig.  141  is  a  diagram 
of  the  connections  of  the  Siemens-Ilgner  system,  from  the  power 
station  to  the  winding  motor,  and  showing  the  method  of  control 
and  the  flywheel  converter. 

With  the  "  Koepe "  system  of  winding  rope,  the  problem  of 
electrical  winding  is  very  much  simplified,  and  it  is  doubtful  whether 
the  complicated  machinery  of  the  Siemens-Ilgner  is  then  necessary. 

A  modification  has  been  developed  by  the  Lahmeyer  Co.,  in 
which  also  a  flywheel  is  employed.  In  this  arrangement  there  are 
two  motors  driving  the  winding  drum,  and  they  receive  current 
partly  from  a  motor  generator,  and  partly  directly  from  the  supply 
service,  the  motor  generator  carrying  a  flywheel,  as  in  the  Siemens- 
Ilgner  arrangement.  To  the  motor  generator  is  added  a  small 


ELECTRICITY  IN   MINING 


Power  Station 


booster,  the  armatures  of  the 
three  machines  being  on  one 
axle,  and  the  peculiarity  of 
the  arrangement  is,  the  pres- 
sure developed  by  the  gene- 
rator of  the  motor  generator 
is  alternately  increased  and 
decreased,  and  reversed,  so  as 
to  be  added  to,  or  subtracted 
from,  the  pressure  of  the 
supply  service.  When  the 
winding  motors  are  at  rest 
before  the  wind  is  started, 
the  pressure  of  the  generator 
side  of  the  motor  generator 
is  equal  to  and  opposite  to 
that  of  the  supply  service,  so 
that  no  current  passes  through 
the  winding  motors.  When 
the  wind  is  to  be  started,  the 
current  in  the  field  magnets 
of  the  motor  generator  is 
weakened,  the  pressure  de- 
livered by  the  motor  gene- 
rator being  therefore  reduced, 
and  a  current  is  then  delivered 
by  the  supply  service,  this 
being  gradually  increased  as 
the  pressure  of  the  motor 
generator  is  decreased.  At 
a  certain  period  of  the  wind 
the  direction  of  excitation 
of  the  motor  generator  is 
reversed,  the  pressure  it 
delivers  being  reversed,  and 
therefore  being  added  to  the 
supply  service,  just  as  when 
two  dynamos  are  connected 
in  series,  this  being  the  actual 
arrangement.  In  this  manner 
the  pressure  rises  from  0  to 
500,  this  being  the  pressure 
of  the  supply  service,  and 
thence  to  1000  volts,  this 
being  the  combined  pressure 


DRIVING   MACHINES   BY  ELECTRICITY         317 

of  the  supply  service  and  the  reversed  motor  generator.  When  the 
current  is  to  be  reduced,  the  excitation  of  the  motor  generator  is 
gradually  again  reduced,  it  again  passes  through  the  zero  point, 
is  again  reversed,  its  pressure  again  gradually  increased,  till  when 
the  wind  is  complete,  the  pressure  is  completely  cut  off  from  the 
winding  motors.  The  flywheel  in  this  case  absorbs  the  power  given 
off  at  a  certain  portion  of  the  wind,  and  restores  it  at  the  moment 
of  starting.  This  arrangement  has  so  far,  the  author  believes,  only 
been  applied  to  mines  where  the  Koepe  balance  system  is  employed, 
and  in  the  special  case  under  notice  at  the  Ligny-des-Aire  mines, 
the  motor  house  is  fixed  on  top  of  the  head  stocks,  the  drums  being 
carried  by  axles  supported  by  the  head  stocks,  and  the  whole  of 
the  wind  being  vertical.  In  this  arrangement  also  the  control  is 
entirely  by  the  engine  man's  lever,  varying  the  excitation  resistance 
in  the  field  coils  of  the  motor  generator,  and  in  the  booster.  The 
variation  in  the  pressure  delivered  by  the  generator  of  the  motor 
generator  is  accomplished  partly  by  varying  the  current  in  its  own 
field  coils,  and  partly  by  varying  the  current  in  the  field  coils  of 
the  booster. 

In  this  arrangement,  and  also  in  the  Siemens-Ilgner,  it  will  be 
noticed  that  the  current  dealt  with  by  the  engine  man,  is  only  the 
small  current  passing  through  the  shunt  coils  of  the  field  magnets  of 
the  motor  generator  or  booster,  or  exciting  dynamo,  so  that  there  is 
no  difficulty  in  constructing  regulators,  worked  by  levers  very  similar 
to  those  used  with  steam  winding,  which  vary  the  resistance,  without 
an  amount  of  sparking  that  cannot  be  easily  extinguished.  Plate 
20c  shows  the  reversing  apparatus  employed  by  the  International 
Electric  Engineering  Co.,  at  the  Waihi  Junction  Mine  in  New  Zealand, 
and  Plate  28s  the  winding  motor  and  brake  at  the  same  mine. 


Westinghouse  System  of  Electrical  Winding: 

The  Westinghouse  Co.  have  worked  out  a  system  of  winding  by 
electricity,  in  which  the  flywheel  storage  system  is  adopted,  but  in 
a  different  manner  to  either  the  Siemens-Ilgner  or  the  Lahmeyer. 
The  Westinghouse  Co.  call  their  arrangement  the  converter  equalizer 
system,  because  a  rotary  converter  is  employed,  not  to  deliver  current 
to  the  winding  motors,  but  to  equalize  the  amount  of  current  taken 
from  the  supply  service,  by  storing  the  current  that  is  not  required 
when  winding  is  not  in  operation,  in  a  flywheel,  the  flywheel 
delivering  the  energy  stored  in  it  to  the  winding  motors,  to  make  up 
the  excess  demand  during  the  periods  of  starting  and  acceleration. 
Where  a  three-phase  high-pressure  transmission  system  is  employed, 
the  three-phase  currents  are  taken  direct  from  the  high  pressure 


ELECTRICITY   IN   MINING 


DRIVING  MACHINES   BY   ELECTRICITY         319 

system  to  the  winding  motors,  transformed  down  if  the  pressure  is 
extra  high  tension.  Branch  circuits  are  taken  from  the  supply 
service  to  a  rotary  converter,  through  a  stepdown  transformer,  and 
the  continuous  current  side  of  the  rotary  converter  is  connected  to  a 
continuous  current  dynamo,  having  a  flywheel  carried  on  its  driving 
axle.  It  is  this  flywheel  which  stores  the  energy,  and  the  dynamo 
which  acts  alternately  as  motor  storing  energy  and  as  generator 
delivering  energy  to  the  high-pressure  three-phase  motors.  The 
arrangement  is  as  follows.  The  winding  drums  are  driven  by  three- 
phase  motors  directly  geared  to  them  by  spur  gearing.  When  the 
winding  motors  are  not  taking  current,  the  whole  of  the  current  that 
would  be  supplied  to  them  passes  to  the  rotary  converter,  and  from 
it  after  conversion,  to  the  flywheel  dynamo,  which  it  runs  as  a  motor, 
the  energy  it  is  delivering  being  stored  in  the  flywheel.  When  the 
wind  is  started,  current  is  delivered  to  the  winding  motors  from  the 
power  service,  and  from  the  rotary  converter.  The  flywheel  dynamo, 
which  is  no  longer  receiving  current,  immediately  commences  to 
generate  current  when  the  wind  commences,  the  flywheel  giving  up 
the  energy  it  has  stored,  and  driving  the  dynamo  as  a  generator. 
The  current  generated  by  the  flywheel  dynamo  is  converted  in  the 
rotary  converter  to  three-phase  currents,  and  thence  after  transfor- 
mation is  delivered  to  the  supply  service,  and  assists  the  currents 
from  the  generating  station  in  supplying  the  winding  motors.  There 
are  a  few  other  details  that  have  been  worked  out  by  the  Westing- 
house  Company.  The  pressure  of  the  flywheel  dynamo  is  controlled 
automatically  from  the  supply  service,  by  a  controller  acting  upon 
its  field  coils.  The  operation  of  winding  is  controlled  by  the  engine 
man  by  means  of  a  lever  moving  over  an  arc,  very  similar  to  that  in 
use  with  steam  winding.  When  the  winding  lever  is  thrown 
forward,  two  operations  take  place,  levers  connected  mechanically 
with  the  engine  man's  lever  move  a  reversing  switch  by  a  link 
motion,  and  they  also  operate  a  liquid  starting  switch.  The  first 
action  as  the  lever  is  moved  forward,  puts  the  reversing  switch 
in  its  proper  position  for  the  wind,  and  it  is  not  until  the  connections 
are  made  in  this  switch  that  the  starting  switch  is  put  in  operation. 
The  starting  switch,  which  consists  of  three  electrodes  in  a  tank 
to  which  the  liquid  is  admitted,  is  then  operated  gradually  in  the 
usual  way,  and  the  wind  and  acceleration  proceeds.  The  winding 
drum  is  provided  with  a  pneumatic  brake,  controlled  by  another 
lever  at  the  engine  man's  left  hand,  and  he  also  has  a  trip 
emergency  lever  close  to  his  foot.  Further,  in  case  of  accident  the 
emergency  brake  is  put  on  automatically  by  an  arrangement  of  trip 
levers  released  by  a  solenoid.  Figs.  142,  143,  and  144  show  the 
connections  and  general  arrangement  of  the  apparatus. 


320 


ELECTRICITY  IN   MINING 


1  g 


DRIVING  MACHINES   BY   ELECTRICITY         321 


322  ELECTRICITY   IN   MINING 


Winding  in  Metalliferous  Mines 

In  the  older  forms  of  metalliferous  mines,  such  as  the  older 
mines  in  Cornwall,  winding  was  very  slow,  and  the  number  of  winds 
per  hour  was  small.  With  the  development  of  the  Eand  mines, 
however,  and  some  other  gold  mines  worked  on  similar  lines,  this 
has  been  changed,  and  winding  is  as  rapid  from  modern  metalliferous 
as  from  coal  mines,  as  much  as  3600  tons  being  brought  to  the 
surface  in  a  shift  of  eleven  hours  and  three-quarters,  and  as  many  as 
92  winds  being  made  per  hour.  The  shafts  of  metalliferous  mines 
differ  from  those  of  coal  mines  in  several  particulars.  In  the  older 
mines  they  are  not  vertical,  but  follow  the  lode,  and  in  some  of  the 
old  Cornish  mines  often  turn  about  in  a  very  peculiar  manner.  Even 
in  modem  mines  on  the  Eand,  and  other  goldfields,  many  of  the 
shafts  are  at  an  inclination  with  the  vertical,  and  in  the  lode. 
Further,  in  metalliferous  mines  it  is  required  to  raise  ore  from  a 
number  of  different  levels,  and  to  be  able  to  stop  the  skip  at  any 
level  from  which  ore  is  ready  to  be  wound.  The  mine  shafts  are 
also  very  much  deeper  than  the  great  majority  of  coal  pits.  Hence 
a  special  arrangement  has  been  introduced  for  winding,  which  is 
practically  an  enlarged  copy  of  the  arrangement  for  straining  the 
rope  in  endless  haulage.  There  are  two  winding  drums,  really 
friction  pulleys,  or  friction  drums  placed  one  in  front  of  the  other, 
driven  by  one  pair  of  engines,  the  connecting  rod  of  the  engine 
driving  the  rear  drum,  and  a  second  pair  of  connecting  rods  driving 
the  front  drums,  in  a  similar  manner  to  the  arrangement  for  driving 
the  wheels  of  a  steam  locomotive.  One  end  of  the  rope  is  attached 
to  one  skip,  is  given  a  few  turns  round  the  front  drum,  then  a  few 
turns  round  the  back  drum,  and  then  it  is  taken  to  the  other  com- 
partment of  the  shaft,  and  thence  to  the  other  skip,  one  skip  descend- 
ing as  the  other  one  rises.  In  addition  to  this,  to  provide  for 
stopping  the  skip  at  different  levels,  the  rope,  before  it  passes  to  the 
other  compartment  of  the  shaft,  is  taken  back  around  a  pulley 
similar  to  the  tightening  pulley  of  an  endless  rope,  and  brought 
forward  again  to  the  shaft.  The  tightening  pulley  is  arranged  to 
run  on  a  pair  of  rails  behind  the  engine  house,  fitted  especially  for 
the  purpose,  the  tightening  pulley  being  held  at  any  point  upon  the 
track,  according  to  the  requirements  of  the  wind.  When  it  is  re- 
quired to  wind  from  a  shallow  level,  the  tightening  pulley  is  run 
back  to  the  full  extent  of  the  track,  that  length  of  rope  being  practi- 
cally wasted.  When  it  is  required  for  the  skip  to  go  to  deeper  levels, 
the  tightening  pulley  is  brought  forward  to  whatever  point  may  be 
required,  the  different  points  being  marked  on  the  track,  and  the 
tightening  pulley  being  moved  by  an  engine  provided  for  the  purpose, 


DRIVING   MACHINES   BY  ELECTRICITY 


323 


the  whole  operation 
being  stated  to  take 
only  a  few  minutes. 
In  this  way  it  is 
arranged  to  wind  from 
each  level  in  turn,  as 
may  be  required.  The 
above  arrangement  is 
for  steam  winding,  and 
is  shown  in  Fig.  145, 
but  it  is  perfectly 
applicable  to  electric 
winding,  one  or  two 
motors  as  may  be 
arranged,  preferably 
two,  being  fixed  to 
drive  the  rear  drum, 
the  forward  drum 
being  driven  by 
parallel  connecting 
rods,  and  another 
motor  being  provided 
for  moving  the  tighten- 
ing drum.  The  winding 
motors  can  be  fed  with 
current  either  directly 
from  a  three  phase 
service,  the  motors 
themselves  being  three 
phase  induction 
motors,  or,  as  would 
probably  be  preferable, 
by  one  of  the  other 
systems  that  have  been 
described,  in  which  a 
flywheel  and  motor 
generator  are  em- 
ployed. In  the  modern 
metalliferous  mines 
also,  the  balance  rope 
system  is  sometimes 
employed,  a  balance 
rope  connecting  the 
bottoms  of  the  two 
skips,  so  that  the  engine 


324  ELECTRICITY  IN   MINING 

or  motor  has  only  to  overcome  the  friction  of  the  rope,  and  to  raise 
the  vertical  load,  plus  the  friction  of  the  skip  wheels  where  the  shaft 
is  inclined. 

In  many  modern  metalliferous  mines,  the  shafts  are  arranged 
vertically  to  strike  the  lode  at  a  certain  point,  and  from  there  what 
are  practically  dip  haulage  engines  run  down  the  incline  formed  by 
the  dip  of  the  lode.  These  can  be  worked  very  conveniently  by 
electric  motors,  just  as  haulage  engines  in  coal  mines  are. 

There  is  one  point,  however,  that  is  to  be  remembered  in  con- 
nection with  metalliferous  mines;  they  are  nearly  always  very 
heavily  watered,  so  that  the  motors  employed  underground  should 
be  constructed  to  stand  water.  Apparently  this  difficulty  has  been 
overcome,  since  at  the  Knight's  Deep  Mine,  which  was  flooded 
during  the  war  for  two  years  and  a  half,  the  motors  which  were 
placed  at  some  of  the  levels  for  driving  pumps,  and  which  were 
drowned  and  under  a  very  heavy  pressure  of  water  during  the  whole 
period,  when  brought  to  the  surface  and  dried,  were  found  to  be  in 
practical  working  order. 

Coal  Cutting  by  Electricity 

The  coal-cutting  machine  is  designed  to  perform  the  office  of 
"holing,"  or  "kirving/'  as  it  is  called  in  the  North,  viz.  cutting 
away  a  space  under  or  over  the  coal,  or  between  two  seams,  when 
there  is  a  parting,  so  that  it  may  be  dislodged  from  its  bed  between 
the  overlying  and  underlying  strata. 


The  Process  of  Holing  or  Kirving 

In  "  holing  "  or  "  kirving  "  by  hand,  the  miner,  lying  on  his  side 
or  in  a  crouching  position,  picks  away  a  certain  quantity  of  either 
the  lower  part  of  the  seam,  the  dirt  between  two  seams,  or  the  dirt 
above  a  seam,  according  how  the  coal  is  to  be  got  out,  and  he  has  to 
cut  away  sufficient  coal,  where  he  is  holing  under  the  coal,  to  allow 
his  own  shoulders  and  his  pick  to  go  under,  and  to  work,  the  result 
being  that  he  cuts  away  a  space,  the  section  of  which  is  an  irregular 
right-angled  triangle,  and  he  makes  in  the  process  a  great  deal  of 
small  coal,  which  has  not  such  a  large  value  as  large,  and  which  at 
the  time  when  coal-cutting  machines  were  first  introduced,  had  only 
a  fraction  of  its  value.  Holing  by  hand  is  necessarily  more  or  less 
irregular,  because  different  men  working  on  the  same  face  work  at 
different  rates.  It  may  happen  that  one  stall  is  not  worked,  on  a 
long  face,  and  this  will  hold  back  the  whole  of  the  face  until  the 
stall  is  brought  up  again.  This  irregularity  leads  to  a  certain 


DRIVING   MACHINES   BY   ELECTRICITY         325 

increase  of  the  danger  from  the  roof  behind  the  coal  face,  because  it 
is  not  possible  to  support  it  so  carefully  as  when  the  face  is  straight. 
Broadly,  there  are  two  principal  methods  of  getting  the  coal,  known 
respectively  as  "  longwall,"  and  "  bord  and  pillar,"  the  latter  method 
being  also  known  sometimes  as  "  pillar  and  stall,"  and  as  "  stoop  and 
room."  There  are  other  systems  of  working  the  coal,  but  the  above 
are  the  principal,  and  they  mark  the  main  differences  in  the  systems. 
In  longwall  there  is  a  long  wall  or  face,  which  may  be  100  yards,  or 
as  much  as  900  yards  long,  and  there  will  be  different  faces  in 
different  districts  of  the  mine,  each  face  moving  outwards  as  the  coal 
is  removed  from  the  shaft  towards  the  boundary  of  the  royalty.  In 
bord  and  pillar,  and  the  other  systems  more  or  less,  the  coal  faces  are 
very  small — from  8  feet  to  20  feet.  In  the  longwall  system  with 
hand  holing,  a  number  of  men  are  working  along  the  whole  face, 
continually  holing  under,  and  continually  removing  the  coal  that  is 
brought  down,  and  other  men  are  working  behind  them,  propping 
the  roof,  and  completing  what  are  called  gate  roads,  roads  leading 
up  to  different  points  in  the  face  at  convenient  distances  apart,  with 
the  rubbish  that  is  removed  from  different  parts  of  the  mine,  pack 
walls  being  formed  in  the  "  goaf,"  as  it  is  termed,  to  support  the  roof, 
which  is  allowed  to  settle  down  on  them.  In  bord  and  pillar  the 
whole  of  the  seam  is  cut  out  in  blocks,  very  much  like  the  squares 
of  a  chess  board,  by  roads  crossing  each  other  at  right  angles,  and  it 
is  these  roads  in  which  the  holing  takes  place  while  the  mine  is 
being  opened  out,  the  pillars  that  are  left  being  afterwards  removed, 
in  what  is  called  working  back,  when  the  roads  have  reached  the 
boundary  of  the  royalty.  It  will  be  seen  that  while  longwall  work- 
ing offers  facilities  for  machine  holing,  bord  and  pillar  do  not  so,  nor 
do  the  other  methods  known  as  "  panel,"  etc.  Machine  holing  has 
been  adopted  almost  entirely  for  longwall  only,  it  being  only  recently 
that  bord  and  pillar  working  is  being  done  by  machine,  though  in 
America  a  modification  of  bord  and  pillar,  in  which  large  "  rooms  " 
are  made  from  18  feet  to  20  feet,  and  in  some  cases  to  60  feet  wide, 
are  also  worked  by  some  of  the  machines  that  will  be  explained. 


Longwall  Coal -cutting  Machines 

There  are  three  forms  of  machines  at  present  on  the  market  for 
coal  cutting  by  electricity  in  longwall  working,  known  respectively 
as  the  disc,  the  bar,  and  the  chain  machines.  The  general  construc- 
tion of  all  of  them  is  very  much  the  same.  There  is  a  rectangular 
frame  formed  by  two  lengths  of  girder  steel,  or  by  castings  joined  by 
cross  pieces  at  the  ends,  and  the  frame  is  either  supported  on  small 
wheels  which  run  on  the  ordinary  tram  rails  of  the  colliery,  these 


326 


ELECTRICITY  IN   MINING 


being  laid  along  the  face  for  the  purpose,  or  on  skids,  contrivances 
very  similar  to  the  runners  of  sleighs.  The  disc,  the  bar,  and  the 
chain  carry  the  cutting  tools,  which  are  intended  to  do  the  work 
the  miner  hitherto  performed  with  his  pick.  The  disc  is  a  wheel 
of  from  3  feet  to  7  feet  in  diameter,  fixed  horizontally,  revolving 
upon  a  vertical  axis,  and  carrying  at  its  periphery  chisel-shaped 
cutting  tools  in  various  ways,  the  endeavour  of  inventors  being  to 
arrange  that  the  tools  shall  be  easily  and  quickly  replaced  when 
worn,  a  blunt  tool  taking  a  considerably  larger  current  than  a  sharp 
tool.  It  is  shown  in  Plates  29A  and  29B.  The  bar  carries  a  larger 
number  of  very  much  smaller  cutters,  shaped  exactly  like  small 
picks,  in  rows  arranged  around  and  along  its  surface.  It  is  shown 


FIG.  146. — Horizontal  Section  showing  the  Action  of  a  Disc  Coal-cutting  Machine. 

in  Plate  29c.  The  chain  carries  chisel-shaped  cutters,  very  similar 
to  those  carried  by  the  disc,  fixed  in  special  links  forming  part  of 
an  endless  chain,  held  between  two  plates  fixed  horizontally,  the 
chain  passing  round  vertical  rollers.  It  is  shown  in  Plate  SOD. 
With  all  three  machines  the  cutting  tools  either  chip  or  scrape 
away  the  clay  which  usually  underlies  the  coal,  or  the  coal  itself, 
as  may  be  arranged,  or  the  dirt  between  two  seams,  etc.,  the  motion 
necessary  for  the  cutting  being  caused  by  the  revolution  of  the  disc, 
and  the  bar  on  their  axes,  and  by  the  chain  running  round  its 
rollers.  Motion  is  imparted  to  the  disc  and  the  bar,  and  the  chain, 
by  one  or  two  electric  motors  carried  at  one  or  both  ends  of  the 
rectangular  frame,  by  means  of  spur  and  bevel  or  worm  gearing.  As 


DRIVING   MACHINES   BY  ELECTRICITY         327 

explained,  the  disc  and  the  chain  revolve  horizontally  on  vertical 
axes,  the  bar  revolves  vertically  on  a  horizontal  axis.  The  disc, 
and  the  chain,  and  the  bar  are  arranged  to  cut  inwards  under  the 
coal,  to  the  depth  necessary  to  allow  the  coal  to  fall,  this  varying 
from  3  feet  to  6  feet,  according  to  the  seam,  and  the  other  methods 
adopted.  With  all  three  machines  there  is  a  small  haulage  drum 
fixed  in  the  front  of  the  machine,  in  the  direction  in  which  the 
cut  is  to  be  made,  and  a  small  galvanized  iron  rope  is  attached  to 
this  drum,  its  other  end  being  secured  to  a  prop  some  distance  in 
front.  The  haulage  drum  receives  motion  from  the  second  motion 
shaft  of  the  gearing,  and  its  rate  of  motion  is  arranged  to  be  varied 
by  different  devices,  according  to  the  rate  of  the  cutting  of  the  coal. 
In  all  cases  the  rope  is  gradually  wound  up  on  the  drum,  and  the 
machine  is  pulled  bodily  forward.  As  the  machine  moves  forward, 


FIG.  147. — Diagram  showing  the  Action  of  a  Disc  Coal-cutting  Machine, 
by  Messrs.  Ernest  Scott  &  Mountain. 

the  disc  and  the  chain  move  forward  under  the  coal,  cutting  the  coal 
or  dirt  away  in  front  of  them  as  they  move,  and  occupying  a  space 
under  the  coal,  equal  to  a  large  portion  of  their  own  surface.  The 
bar  also  cuts  away  the  coal  or  the  dirt  as  the  machine  is  moved 
forward,  but  it  only  itself  occupies  a  very  small  space,  the  bar  being 
only  a  few  inches  in  diameter  as  compared  with  the  disc  and  the 
chain,  which  occupy  several  feet.  The  disc  runs  at  from  20  revolu- 
tions to  70  revolutions  per  minute,  while  the  bar  runs  from  300 
revolutions  to  500  revolutions  per  minute.  The  depth  of  the  under- 
cut is  regulated  by  the  length  of  the  bar,  the  length  of  the  chain,  and 
the  diameter  of  the  disc,  while  the  width  of  the  cut  is  regulated  by 
the  space  from  top  to  bottom  occupied  by  the  cutters,  and  is  usually 
about  4  inches.  Figs.  146  and  147  show  the  working  of  disc 
machines. 

Heading  Machines 

For  bord  and  pillar  working,  for  driving  headings  and  so  on, 
another  form  of  chain  machine  is  employed,  an  importation  from 
America,  in  which  a  chain  similar  to  that  with  the  longwall  header, 
and  carrying  cutters  very  similar  to  them,  but  with  a  very  much 


ELECTRICITY  IN   MINING 

longer  chain,  is  employed.  The  chain  is  carried  on  a  long  rectangular 
frame,  supported  upon  a  stouter  frame  resting  on  wheels  or  skids, 
similar  to  but  larger  than  that  of  the  longwall  machines,  and  is  pre- 
sented with  one  of  its  small  ends  to  the  coal.  The  machine  is  shored 
up  close  to  the  face,  the  chain  is  set  in  motion  around  the  frame,  and 
the  frame  is  forced  outwards  under  or  in  the  coal,  and  it  cuts  a 
groove  the  usual  width,  about  4  inches  to  whatever  depth  under  may 
be  arranged,  and  about  2  feet  8  inches  along  the  face.  After  one 
2  feet  8  inches  has  been  cut,  the  frame  is  run  back,  the  machine 
is  moved  over  to  the  right  or  left,  as  may  be  arranged,  another  cut  is 
made,  connected  with  the  first,  this  being  followed  by  a  third  cut, 
and  so  on,  till  the  width  of  the  heading  or  the  room  has  been  cut 
across.  In  America  the  coal  is  got  by  a  succession  of  rooms  18  feet, 
20  feet,  and  sometimes  as  much  as  60  feet  wide,  with  pillars  of  coal 
between  the  rooms,  and  it  is  usual  to  cut  across  one  room,  and  then 
move  the  machine  to  the  next  room  while  the  coal  is  got  down  in  the 
first,  and  so  on. 


The  Rotary  Heading  Machine 

There  is  one  other  form  of  coal-cutting  machine  known  as  the 
rotary  heading  machine,  employed  in  opening  out  collieries,  where 
it  is  required  to  get  the  coal  very  quickly.  It  consists  of  a  bar 
pivoted  at  the  middle  of  its  length,  and  carrying  at  each  end  arms 
with  cutting  tools.  When  the  machine  is  at  work,  the  bar  is  pushed 
up  to  the  face  of  the  coal,  and  is  rotated  about  its  centre,  the  cutting 
tools  on  the  arms  at  its  end  cutting  into  the  coal,  and  forming  an 
annular  groove,  the  width  of  the  cutting  tools  and  with  the  diameter 
of  the  rotating  bar.  When  the  cutting  tools  have  cut  in  as  far  as  they 
will  go,  the  bar  is  run  back,  the  whole  machine  is  moved  to  the  rear, 
and  the  solid  cylindrical  core  of  coal,  left  inside  the  annular  groove, 
is  brought  down  by  blasting  in  the  usual  way.  The  rotating  bar  is 
pivoted  on  an  axle  geared  to  the  crankshaft  of  a  double-cylinder 
compressed  air  engine,  gearing  being  also  provided  to  push  the  bar 
bodily  forward  as  the  cut  proceeds,  the  whole  being  mounted  on  a 
substantial  bed  plate,  with  uprights  for  carrying  the  bearings  of  the 
crankshaft  and  of  the  axle-rotating  bar,  the  bed  plate  being  mounted 
on  wheels  or  skids,  as  may  be  arranged.  The  machine  has  been 
adapted  for  electric  driving  by  fixing  an  electric  motor  in  place  of  the 
double-cylinder  compressed  air  engine,  and  gearing  the  axle  of  the 
motor  to  the  rotating  axle  of  the  bar,  by  spur  gearing. 


PLATE  26 A.— Electrically  Driven  Three  Throw  Ram  Pump,  arranged  for  mining  on 
the  Mine  Rails,  made  by  Messrs.  M.  B.  Wild  &  Co. 


PLATE    26s. — Electric    Mine  Loco,   as   employed  in    America    and  on  the 
Continent,  made  by  the  Jeffrey  Co. 

[To  face  p.  328. 


DRIVING   MACHINES   BY   ELECTRICITY         329 


Motors  employed  with  Coal  -cutting  Machines 

So  far  series-wound  and  three-phase  motors  only  have  been 
employed  for  driving  coal-cutting  machines.  The  plan  adopted  in 
the  great  majority  of  the  machines  is,  the  motor  is  fixed  at  one  end  of 
the  rectangular  frame,  the  gearing  being  in  the  centre,  and  the  disc, 
bar,  or  chain  gearing  at  the  other  end,  the  small  haulage  drum  being 
in  the  neighbourhood  of  the  motor.  In  the  Diamond  machine,  and 
in  the  Brush-Kirkup,  two  motors  are  employed,  fixed  one  at  each  end 
of  the  rectangular  frame,  the  gearing  being  in  the  middle,  and  the 
disc  working  also  at  the  middle  of  the  frame.  With  the  bar  machine 
and  the  chain  "  longwall  "  machine,  gearing  is  provided  for  moving 
the  bar  and  the  plates  between  which  the  chain  moves  from  a 
position  in  line  with  the  body  of  the  machine  to  the  position  at 
right  angles  to  the  machine,  that  it  has  to  occupy  when  cutting  the 
coal,  the  gearing  in  this  case  being  enclosed  inside  dust-proof  cases. 
In  the  case  of  the  bar  machine  also,  a  reciprocating  motion  is  given 
to  the  bar,  so  that  the  cutting  tools  do  not  cut  opposite  the  same 
place  at  each  part  of  the  revolution.  In  the  Peake  machine,  which 
is  an  improvement  of  the  early  Goolden  bar  machine,  the  bar,  which 
is  square  in  section,  is  mounted  directly  on  the  end  of  the  axle  of  the 
armature,  the  gearing  being  thus  dispensed  with. 

The  motors  provided  for  coal-cutting  machines  range  from 
nominal  25  H.P.  up  to  35  H.P.  ;  they  take  from  16  H.P.,  under  the 
most  favourable  circumstances,  up  to  as  much  as  50  H.P.,  this  latter 
being  only  for  a  short  time.  Where  two  motors  are  employed,  the 
power  is,  of  course,  divided  between  the  two,  and  they  are  connected 
in  series. 


Delivering  the  Current  to  Coal-cutting  Machines 

As  explained  in  an  earlier  part  of  the  book,  the  supply  cables  are 
brought  to  switchboxes  at  the  ends  of  the  gate  roads,  and  from  these 
switchboxes  flexible  cables  are  taken  to  the  motors.  It  is  an 
important  point  in  connecting  the  cables  to  the  motors,  that  the 
connection  to  the  motor  should  be  made  first,  and  that  to  the  switch- 
box  last.  Connection  in  both  cases  should  be  by  a  simple  strong 
form  of  plug,  arranged  to  push  into  a  socket,  and  the  switchbox 
should  be  so  constructed  that  it  is  not  possible  to  pull  out  the  plug 
unless  the  switch  is  open.  If  the  plug  is  pulled  out  with  the  switch 
closed,  sparks  will  pass,  and  in  case  of  gas  being  present  the  conse- 
quences may  be  serious. 

With  continuous-current  motors  it  is  necessary,  as  in  other  cases, 
to  provide  a  starting  resistance  and  starting  switch.  The  starting 


330  ELECTRICITY  IN   MINING 

resistance  is  made  in  various  forms,  the  wire  of  which  it  is  formed 
being  covered  with  asbestos  and  protected  in  various  ways,  and  the 
whole  thing  should  be  arranged  that  any  sparking  which  takes  place 
should  be  inside  an  enclosure,  from  which  gas  is  excluded.  The 
commutator  of  the  motor  should  also  either  be  totally  enclosed  or 
inside  a  gauze  enclosure,  and  the  attendants  should  be  warned  to 
keep  the  gauze  free  from  coal  dust,  and  not  to  open  the  case  of  the 
motor  unless  the  current  is  switched  off.  With  three-phase  motors  it 
is  usual  to  employ  the  squirrel-cage  type,  as  this  avoids  the  necessity 
of  any  starting  resistance,  but  there  is  a  difficulty  with  this  type  in 
making  the  machine  cut  its  way  into  the  coal.  The  difficulty,  however, 
is  not  a  very  serious  one,  as  the  machine  often  has  a  run  along  the 
face  of  several  hundred  yards,  and  it  is  only  required  to  make  a  place 
for  it  at  starting.  There  is  also  the  difficulty  with  the  disc  machine 
when  run  by  a  three-phase  motor,  if  the  coal  settles  down  upon  the 
disc,  in  obtaining  power  to  free  it.  Mr.  Eoslyn  Holiday,  who  has 
done  a  good  deal  in  this  matter,  states  that  he  is  able  to  get  over  both 
difficulties  by  switching  the  current  on  and  off  several  times,  a  certain 
torque  being  obtained  each  time  the  current  is  switched  on,  and  the 
coal  being  gradually  cut  into,  or  the  disc  being  gradually  freed.  The 
three-phase  motor  has  the  advantage  that  the  switch,  which  of  course 
must  be  triple-pole,  can  be  completely  enclosed  in  a  gas-tight 
chamber,  the  rods  working  it  passing  through  gas-tight  glands. 


Drilling  by  Electricity 

Drilling  is  carried  on  very  extensively  in  all  mining  work,  for 
the  purpose  of  providing  holes  in  which  charges  of  powder,  or 
dynamite  or  other  explosives  are  placed,  to  bring  the  mineral  down. 
In  coal  mining,  in  the  great  majority  of  cases,  after  a  space  has  been 
provided  for  the  coal  to  fall  by  undercutting,  as  explained  in  con- 
nection with  coal-cutting  machines,  some  force  is  necessary  to  break 
the  coal  away  from  the  strata  overlying  it,  or,  where  the  cut  has  been 
made  above,  to  separate  it  from  the  underlying  strata,  and  this  force  is 
provided  by  blasting.  Attempts  have  been  made  from  time  to  time  to 
do  away  with  blasting.  Mr.  W.  E.  Garforth,  at  Messrs.  Pearson  & 
Knowles's  collieries  at  Nbrmanton,  has  succeeded  in  doing  away  with 
blasting,  by  undercutting  just  to  the  level  of  the  cleat  of  the  coal. 

The  coal  seam  in  this  case  being  thick  (4  feet  6  inches),  an  undercut 
of  6  feet  provides  sufficient  weight  to  break  the  coal  away  at  the  cleat, 
without  any  other  force.  Lime  cartridges  and  hydraulic  cartridges 
have  also  been  introduced,  the  operation  in  each  case  being  the 
expansion  of  the  cartridge  in  the  hole  provided  for  it,  the  force  exerted 
by  the  expanding  material  breaking  the  coal  down.  But  both  of 


DRIVING  MACHINES  BY  ELECTRICITY         331 

them  require  that  holes  shall  be  drilled,  and  in  the  great  majority  of 
coal  mines  and  in  all  metalliferous  mines  blasting  is  still  in  use. 
The  hole  drilled  is  several  feet  in  length,  and  from  1J  to  2  inches  in 


diameter,  and  the  number  of  holes  and  their  depth  will  depend  upon 
the  nature  of  the  mineral,  the  cohesive  force  holding  it  to  the  other 
strata,  the  thickness  of  the  seam,  and  so  on. 


332 


ELECTRICITY  IN   MINING 


There  are  two  forms  of  drilling  machines — the  rotary  and  the 
percussion  drill.  The  rotary  drill  is  applicable  to  comparatively  soft 
material,  the  percussive  drill  being  employed  for  hard  rock.  The 

rotary  drill  is  easily  driven  by 
an  electric  motor,  as  shown  in 
Figs.  148  and  149,  and  Plate  22D, 
all  that  is  necessary  being  the 
provision  of  a  motor  of  about 
2  H.P.  geared  to  the  drill,  the 
whole  being  held  on  a  tripod  or 
telescopic  arrangement,  as  may 
be  arranged. 

Driving  the  percussion  drill, 
however,  is  by  no  means  such 
an  easy  matter.  The  favourite 
method  adopted  by  Messrs. 
Siemens  and  others  is,  an  electric 
motor,  which  can  be  either  carried 
by  the  drill  carriage,  or  in  a  box 
lying  on  the  floor  of  the  mine, 
the  power  being  then  conveyed 
to  the  drill  by  a  flexible  shaft, 
compressed  springs  placed  in  the 
rear  of  the  drill  carriage,  the  drill 
carriage  being  released  when  the 
springs  have  been  compressed  to 
a  certain  pressure,  and  the  drill 
being  thrown  forward  by  the  force 
of  the  expanding  springs.  The 
Siemens  drill  is  shown  in  Fig.  150. 
The  case  containing  the  motor,  for 
standing  on  the  floor  and  driving 
by  flexible  shaft,  is  shown  in 
Fig.  151  for  continuous  current, 
and  in  Fig.  152  for  three  phase. 
There  are  springs  also  in  front 
of  the  carriage  which  force  the 
drill  back  after  the  blow  has  been 
struck,  and  there  is  the  usual 
rifling  arrangement,  which  comes 
into  operation  as  the  drill  returns, 
by  which  rotation  is  caused.  In 
another  form  of  drill  made  by  the  Denver  Engineering  Co.,  Colorado, 
known  as  the  box  drill,  air  is  compressed  inside  a  cylinder  form- 
ing the  drill  carriage,  the  drill  being  held  in  front,  and  working 


DRIVING   MACHINES   BY  ELECTRICITY         333 

in  guides  as  usual,  and  the  compression  of  the  air  being  accom- 
plished by  an  electric  motor  attached  to  the  back  of  the  drill. 
The  construction  of  the  drill  is  shown  in  Figs.  153  and  154.  The 
compressed  air  in  this  drill  takes  the  place  of  the  springs  in 
the  other  forms  described.  In  the  Marvin  electric  percussion 
drill,  made  by  the  Sandicroft  Foundry  Co.,  another  principle  is 
introduced.  Motion  is  communicated  to  the  drill  by  means  of 


FIG  151. — Motor  Case  for  Continuous-current  Motor  to  be  used  with 
Siemens'  Electric  Drills. 

a  solid  steel  plunger,  round  which  two  coils  of  wire  are  fixed, 
electric  currents  passing  through  the  coils.  The  plunger  is  pulled 
back  by  the  current  passing  in  one  coil,  and  in  receding  it  com- 
presses a  strong  spiral  spring  in  the  rear.  It  is  forced  forward 
by  the  current  in  the  other  coil,  aided  by  the  force  of  the  ex- 
panding spiral  spring.  This  arrangement  is  a  modification  of  an 


FIG.  152. — Three-phase  Motor  Case  to  be  used  with  Siemens'  Electric  Drills. 

earlier  form  of  the  same  drill,  in  which  alternating  currents  were 
employed,  and  in  which  it  was  found  that  the  plunger  heated 
very  considerably.  In  the  Marvin  Sandicroft  drill  a  pulsating 
current  is  employed,  furnished  by  a  special  generator.  The  gene- 
rator is  a  continuous-current  machine,  separately  excited  from 
the  lighting  service  or  any  convenient  supply,  and  arranged,  by 
means  of  a  special  commutator,  in  place  of  the  usual  segmental 


334 


ELECTRICITY  IN  MINING 


commutator,   to   furnish   a  pulsating    current,   which    is  delivered 
successively  to  the  different  coils  of  the  drill.     The  coils  and  the  drill 


DRIVING   MACHINES   BY  ELECTRICITY         335 

are  insulated  with  mica  only.  There  is  the  usual  rifle  arrangement 
for  rotating  the  drill.  The  Sandicroft  Marvin  electric  percussion 
drill  is  shown  in  Fig.  155. 

With  percussion  drills  the  depth  of  hole  required  is  obtained 
by  successive  lengths  of  drill.  A  short  drill  is  first  inserted,  and 
allowed  to  drill  to  its  full  extent.  It  is  then  withdrawn,  and  a 
longer  drill  inserted,  and  so  on. 


Coal  Cutting  by  means  of  Drilling  Machines 

There  is  a  field  for  the  electric  percussion  drill  as  an  aid  to  coal- 
cutting  machines.  As  explained,  in  connection  with  coal  cutting, 
in  bord  and  pillar  and  other  work,  it  is  difficult  to  economically 
employ  either  the  bar,  the  disc,  or  the  chain  machine,  except  under 
the  conditions  ruling  in  America,  where  the  chain  heading  machine 
is  employed  in  large  "rooms."  In  addition,  in  narrow  work,  as 
mining  men  term  it,  where  narrow  roads  or  headings  are  being  cut, 
holing  under  the  coal  is  not  sufficient  to  bring  it  down  unless  a 
considerable  amount  of  blasting  is  employed,  and  it  is  the  practice 
to  "  nick "  the  coal,  as  it  is  termed,  on  one  side,  and  sometimes  on 
both,  the  "  nick "  being  a  vertical  cut  sufficient  to  free  the  coal  on 
that  side.  The  chain  heading  machine,  described  on  page  327,  has 
been  adapted  in  America  also  as  a  "  nicking "  machine,  a  band  saw 
being  fixed  on  the  side  of  the  machine,  worked  by  the  electric  motor 
and  arranged  to  cut  a  vertical  "nick,"  as  described.  But  this 
arrangement  is  not  suitable  for  narrow  work,  and  it  absorbs  a  good 
deal  of  time  in  fitting  up  the  "  nicking  "  tool,  while  the  tool  itself 
is  something  more  to  be  carried  about.  During  recent  years,  the 
compressed  air  percussion  drill  has  been  adapted  for  the  work  under 
the  name  of  the  "  Champion."  In  this  apparatus  a  percussion  drill 
is  carried  by  means  of  a  universal  joint  upon  a  vertical  telescopic 
support,  arranged  to  be  fixed  quickly  in  any  coal  face,  heading,  etc., 
and  the  drill  is  employed,  instead  of  drilling  a  hole,  to  sweep  out  an 
arc,  the  radius  of  which  is  the  distance  between  the  front  end  of  the 
drill  and  the  universal  joint,  the  drill  swivelling  horizontally  or 
vertically,  or  at  any  angle  round  the  pivot.  For  undercutting,  a 
succession  of  arcs  are  swept  out,  one  above  the  other,  and  the  cut 
is  made  deeper  by  using  longer  and  longer  drills,  until  the  coal  is 
sufficiently  undercut  for  the  purpose.  Nicking  is  carried  out  by 
arranging  the  drill  to  sweep  a  vertical  arc,  and  by  lengthening  the 
drill  till  the  cut  is  sufficiently  deep.  There  is  no  reason  why 
electric  percussion  drills  should  not  be  employed  in  the  same 
manner,  and  they  should  be  more  convenient  and  more  economical 
than  the  compressed  air  drill.  It  will  be  observed  that  the  above 


336  ELECTRICITY   IN   MINING 

arrangement  overcomes  all  the  difficulties  that  have  been  mentioned 
in  connection  with  coal  cutting  by  machines.  The  drill  is  light, 
and  takes  to  pieces,  and  can  be  easily  transported,  while  coal-cutting 
machines  weigh  from  fifteen  cwt.  to  two  tons,  and  are  not  easily 
transported. 


Electrically  Driven  Fans 

The  fan  has  now  practically  taken  the  place  of  the  furnace  in 
all  coal  mines,  and,  where  ventilation  is  attempted,  is  used  in  all 
metalliferous  mines.  The  fan  employed  in  mines  varies  in  form, 
but  essentially  it  consists  of  a  number  of  blades  assembled  round 
a  shaft,  and  when  the  blades  are  whirled  round,  the  air  enters  the 
centre  of  the  fan  and  is  expelled  at  the  ends  of  the  blades.  Plate 
30A  shows  a  Heenan  fan  without  its  case ;  Plate  30s  a  fan  in  its  case, 
driven  by  an  electric  motor ;  and  Plate  30c  a  fan-house,  away  from 
the  power  station,  with  a  fan  electrically  driven  by  three-phase 
currents. 


Description  of  Different  Forms  of  Fans 

There  are  two  principal  forms  of  fans,  one  of  which  only  has  been 
employed  up  to  the  present  for  mining  work.  The  author  knows 
them  as  the  screw-blade  fan,  and  the  centrifugal  fan.  The  screw- 
blade  fan  has  not  hitherto  been  employed  in  mines,  but  there  appears 
to  be  no  reason  why  it  should  not  be  employed  in  certain  cases 
where  only  a  small  pressure  is  required,  as,  say,  to  divert  an  air 
current  through  certain  portions  of  the  workings.  The  screw-blade 
fan,  or  propeller  fan,  as  it  is  sometimes  called,  merely  acts  in  air, 
as  the  screw  of  a  ship  does  in  water,  or  an  ordinary  screw  does  in 
wood  or  metal.  When  a  body  of  metal  having  the  screw  formation 
is  rotated,  one  of  three  things  may  happen ;  it  may  go  forward  itself, 
as  when  the  screw  moves  into  wood  or  metal ;  it  may  move  the  object 
to  which  it  is  attached  through  the  medium  in  which  it  is  screwing, 
as  when  a  ship  is  moved  through  the  water  by  the  rotation  of  its 
screw ;  or,  if  the  screw  is  stationary,  and  the  object  to  which  it  is 
attached  is  stationary,  it  must  move  the  medium  in  which  it  is 
rotating ;  and  this  is  what  the  propeller  fan  does.  If  fixed  in 
a  window  or  a  door,  or  a  partition  between  bodies  of  air,  when 
rotated  it  transports  the  air  from  the  one  side  of  the  partition  to 
the  other.  It  does  not  create  any  appreciable  difference  of  pressure, 
and  therefore  is  of  no  service  for  driving  a  powerful  current  of  air 
through  a  coal  mine ;  but  in  many  of  the  workings  of  metalliferous 
mines,  where  ventilation  is  often  so  difficult,  and  in  cases  even  in 


PLATE  27A. — Electrically  Driven  Endless  Rope  Haulage  Plant,  with  Spur  Gearing 
by  Messrs.  M.  B.  Wild  &  Co. 


PLATE  2?B. — Dip  Haulage,  or  small  Winding  Plant,  Electrically  Driven,  made  by 
Messrs.  M.  B.  Wild  &  Co. 


PLATE  27c. — Main  and  Tail  Haulage  Plant,  driven  by  Three  Phase  Motor,  as  made 
by  the  Westinghouse  Co. 

[To  face  p.  336. 


DRIVING  MACHINES   BY   ELECTRICITY         337 

coal  mines  where  it  is  sometimes  inconvenient  to  lead  the  air  current 
through  a  duct,  the  propeller  fan,  if  placed  in  a  suitable  position, 
could  either  force  air  into  the  district  or  the  room  to  be  ventilated, 
or  could  withdraw  the  foul  air,  the  pressure  it  creates  being  quite 
sufficient  for  a  great  many  of  these  purposes.  The  propeller  fan 
has  a  large  vogue  in  ventilating  buildings,  offices,  etc.,  though  its 
application  in  many  instances  is  exceedingly  crude,  it  being  supposed 
that  for  ventilation  all  that  is  necessary  is  to  stir  up  the  air  in  the 
middle  of  a  room,  for  instance.  The  propeller  fan  is  so  particularly 
useful  for  many  cases  where  intelligently  applied,  because  it  is  so 
easily  adapted  for  driving  by  small  electric  motors.  It  is  wise, 
however,  when  purchasing  fans  driven  by  electric  motors,  to  discount 
very  largely  the  statements  sometimes  made  in  makers'  catalogues — 
that  they  will  deliver  so  many  cubic  feet  of  air  per  minute.  The 
statement  is  quite  correct,  providing  that  the  air  is  quite  untrammelled, 
as  when  the  fan  is  placed  in  the  middle  of  a  large  open  space,  or  a 
large  room ;  but  where  ventilation  is  required,  there  is  a  resistance 
always  set  up  to  the  passage  of  the  air,  as  will  be  explained,  and 
then  the  quantity  of  air  moved  by  the  propeller  fan  is  very  con- 
siderably reduced. 

The  other  form,  which  has  been  so  largely  used  in  mines, 
is  the  centrifugal  fan.  This  consists  of  a  number  of  blades  assem- 
bled round  a  central  space  very  much  after  the  manner  of  the 
centrifugal  pump,  the  blades  being  enclosed  between  discs,  and  the 
peripheries  of  the  discs  sometimes  being  open  to  the  atmosphere, 
and  sometimes  enclosed,  so  as  to  deliver  the  air  to  some  form  of 
funnel.  As  with  the  centrifugal  pump,  when  the  blades  of  the  fan 
are  rotated,  difference  of  pressure  is  created  between  the  atmosphere 
at  the  periphery  of  the  fan  and  the  centre,  the  air  thence  passing  into 
the  centre  of  the  fan  and  being  whirled  outwards  to  the  periphery, 
thence  to  the  atmosphere  directly,  or  through  the  funnel,  etc.  In 
the  early  forms  of  fan,  the  blades  were  simply  radial,  but  it  was 
found  that  a  good  deal  of  power  was  lost  by  the  eddy  currents  set 
up  between  the  blades,  and  so,  as  with  the  centrifugal  pump,  later 
forms  have  blades  curved  in  the  direction  of  rotation,  and  the  air  is 
guided  more  or  less  in  its  passage  to  the  outer  atmosphere.  There 
is  one  peculiar  feature  also  about  all  fans,  and  that  is,  that  in  the 
centre  of  the  fan  there  is  a  current  of  air  in  the  opposite  direction 
to  that  at  the  periphery,  this  being  the  equivalent  of  the  back 
pressures  electrical  engineers  are  accustomed  to  deal  with  in  their 
apparatus,  but  meaning  the  expenditure  of  more  power  in  order 
to  deliver  the  same  quantity  of  air.  In  the  Capel  fan,  the  air  is 
guided  on  something  the  same  lines  as  the  water  in  the  modern 
high  lift  centrifugal  pump,  with  the  result  that  it  is  claimed  that 
the  velocity  of  the  air  is  completely  got  rid  of,  and  is  reduced 

z 


338  ELECTRICITY   IN   MINING 

to  an  inappreciable  amount  before  it  issues  into  the  atmosphere, 
an  evase  chimney  in  which  the  orifice  is  gradually  expended 
helping  this  effect. 


The  Sirocco  Fan 

In  the  "  Sirocco "  fan,  made  by  Messrs.  Davidson  of  Belfast,  a 
new  line  has  been  struck.  The  appearance  of  the  fan  is  very  much 
like  that  of  some  of  the  old  water  wheels,  with  a  number  of  shallow 
buckets  surrounding  the  wheel.  In  place  of  a  small  number  of  long 
blades,  the  Sirocco  has  a  large  number  of  very  short  blades,  curved 
towards  their  outer  ends  in  the  direction  of  the  motion  of  rotation, 
and  the  blades  are  made  very  much  longer  axially  than  with  the 
other  forms  of  fan.  The  short  blades  are  fixed  between  two  annular 
discs,  the  inner  discs  being  connected  mechanically  with  the  driving 
arrangement,  and  the  internal  space,  where  the  air  space  enters,  being 
many  times  larger  than  in  the  other  types  of  fan.  The  air  also  is 
not  confined  in  any  way  on  its  egress,  the  outlet  being  also  very  large. 
Messrs.  Davidson  claim  to  be  able  to  handle  a  very  much  larger 
quantity  of  air  with  a  given  size  and  weight  of  Sirocco  fan  than 
is  possible  with  other  forms. 

In  all  cases  the  fan  is  a  machine  for  creating  air  pressure,  or  for 
transporting  air  from  one  point  to  another.  In  the  case  of  the 
propeller  fan,  as  explained,  the  air  is  merely  transported  from  one 
side  of  a  partition  to  another,  and  very  small  difference  of  pressure 
created.  With  centrifugal  fans,  however,  comparatively  high  pressures 
for  air  are  created,  as  much  as  8  inches  of  water  gauge  being  created 
by  some  of  the  Capel  fans.  Air  pressure  is  so  small  that  it  is  measured 
in  inches  of  water  gauge — that  is,  the  pressure  equal  to  the  weight 
of  one  or  more  cubic  inches  of  water.  The  inch  water  gauge  equals  a 
pressure  of  0'55  oz.  per  square  inch,  so  that  a  pressure  of  6  inches, 
which  is  considered  very  high,  is  only  about  3J  oz.  per  square  inch. 
This  pressure,  however,  is  quite  sufficient  to  overcome  the  resistance 
of  every  coal  mine,  and  in  the  majority  of  cases  in  British  coal  mines, 
very  much  lower  pressures,  in  the  neighbourhood  of  2  inches  water 
gauge,  are  found  sufficient.  British  mining  engineers  prefer  to  work 
with  low  pressures,  because  they  say  that  as  their  coals  are  many  of 
them  constantly  giving  off  gas,  it  is  better  for  the  gas  to  come  away 
freely,  and  to  be  carried  by  the  ventilating  current  into  the  outer 
atmosphere,  than  for  it  to  be  compressed  within  the  coal  and  held 
there  by  a  powerful  air  pressure,  only  to  come  out  with  considerable 
force  should  the  air  pressure  at  any  moment  be  lowered  by  accident 
to  the  ventilating  apparatus. 

It  is  usual  in  coal  mines  to  place  the  fan  at  the  top  of  the  upcast 


DRIVING   MACHINES   BY   ELECTRICITY         339 

shaft,  the  shaft  being  closed  in,  and  what  is  called  a  fan  drift  being 
led  from  it  directly  to  the  centre  of  the  fan,  the  periphery  of  the 
fan  being  opened  either  directly  or  indirectly  to  the  atmosphere. 
The  motion  of  the  fan  creates  a  lowered  pressure  at  its  entrance — 
that  is  to  say,  a  difference  of  pressure  between  the  atmosphere  at 
the  top  of  the  downcast  shaft  and  the  air  entering  the  fan.  This 
difference  of  pressure  is  similar  in  every  respect  to  the  difference  of 
pressure,  which  causes  a  transference  of  electricity  through  a  con- 
ductor. In  fact,  the  ventilation  of  a  coal  mine  is  similar  in  almost 
every  respect  to  the  distribution  of  electricity  on  the  two-wire  system. 
There  are  two  main  roads  in  the  mine,  leading  from  the  downcast  and 
the  upcast  shaft  respectively,  and  these  are  connected  by  other  roads 
leading  to  the  faces,  to  the  stables,  etc.,  in  such  a  manner  that  the  air 
passes  from  the  intake  road,  the  one  leading  from  the  downcast,  across 
the  face,  or  the  portion  of  the  mine  to  be  ventilated,  to  the  return 
airway,  that  leading  to  the  upcast.  The  fan  is  required  to  create  a 
sufficient  difference  of  pressure  to  force  a  sufficient  quantity  of  air 
through  the  main  roads  and  the  workings  to  comply  with  the  Coal 
Mines  Eegulation  Act.  The  air  in  its  passage  through  the  roadways, 
etc.,  rubs  on  the  sides,  and  creates  friction,  the  friction  being  directly 
in  proportion  to  the  extent  of  the  surface — that  is  to  say,  to  the 
lengths  of  the  roads,  and  to  the  size  of  the  roads.  A  certain  power  is 
required  to  move  the  air  through  the  roads,  the  power  being  directly 
in  proportion  to  the  square  of  the  velocity,  and  to  the  friction,  etc.  A 
certain  power  is  also  required  to  create  the  difference  of  pressure 
between  the  two  sides  of  the  fan.  The  whole  thing  resolves  itself 
into  a  certain  power  being  required  to  drive  the  fan,  and  this  may  be 
supplied  by  an  electric  motor,  which  is  preferably  of  the  shunt- wound 
continuous-current  form.  Three-phase  motors  are  employed  in 
driving  fans,  but  a  difficulty  has  arisen  in  connection  with  their 
employment.  The  fan  is  often  required  to  vary  its  speed,  not  for  a 
few  minutes,  but  for  days  together,  owing  sometimes  to  changes  in  the 
barometric  pressure  of  the  atmosphere,  and  sometimes  to  the  fact 
that  the  pressure  is  lowered  at  week  ends,  when  there  is  no  one  in 
the  pit.  The  variation  in  speed  is  not  great.  Fans  run  at  from  40 
revolutions  per  minute  up  to  300  revolutions,  and  a  variation  of  1  or 
2  revolutions  of  a  fan  running  at  40,  and  the  equivalent  on  the  higher 
speed  fans,  is  all  that  is  required.  This  is  easily  obtained  with  a 
shunt-wound  continuous-current  motor,  with  very  little  waste,  by 
varying  the  strength  of  the  field  current,  but  it  is  not  so  easily 
obtained  without  waste  with  a  three-phase  motor,  because  a  resistance 
must  be  put  in  the  rotor  circuit  of  sufficient  size  to  accommodate 
the  whole  of  the  rotor  current,  and  the  heat  generated  in  that 
resistance  is  wasted. 

Electricity  cannot  compete  with  steam  for  driving  fans  where  the 


340  ELECTRICITY   IN   MINING 

boiler  is  close  to  the  fan  engine,  since  the  fan  engine  itself  is  an 
absolutely  constant  load,  subject  to  the  variations  mentioned  above, 
and  therefore  any  economies  that  can  be  effected  in  an  engine  driving 
the  generator  at  an  electric  power  station,  can  be  applied  directly  to 
the  engine  driving  the  fan  itself.  There  are,  however,  many  cases 
where  a  fan  is  placed  at  a  distance  from  the  boilers.  It  may  be 
required  to  ventilate  a  pit  where  there  is  no  steam,  and  an  electrically 
driven  fan  then  comes  in  most  conveniently.  Also,  it  often  happens 
that  parts  of  the  workings  are  difficult  to  arrange  for  ventilation,  and 
in  those  cases  it  should  be  very  convenient  indeed  to  place  a  fan  in 
such  a  position  that  it  can  be  driven  by  an  electric  motor  without 
danger,  and  can  either  force  or  suck  air  through  the  district  to  be 
ventilated.  The  electrical  fan,  in  fact,  should  be  as  useful  to  mine 
ventilation  in  coal  mines,  and  very  much  more  so  in  metalliferous 
mines,  as  the  booster  is  to  the  electric  power  distribution. 


Power  required  for  Driving  Fans 

As  explained,  the  power  required  by  a  fan  is  expended  in  creating 
the  necessary  pressure  between  two  surfaces,  as  the  top  of  the  down- 
cast and  the  top  of  the  upcast  pits  of  a  coal  mine ;  between  the  ends 
of  a  road  leading  to  a  portion  of  the  workings,  and  so  on ;  and  its 
necessity  arises  from  the  fact  that  air,  like  water,  when  it  is  driven 
through  any  pipe  or  duct,  or  the  roads  of  a  mine,  rubs  upon  the  sides, 
roof,  floor,  etc.,  and  in  rubbing  creates  friction,  which  absorbs  power 
to  overcome  it.  The  power  required  is  measured  by  the  product  of 
the  quantity  of  air  passing  per  minute,  multiplied  by  the  pressure 
between  the  two  sides  of  the  fan,  the  pressure  being  expressed  in 
the  weight  of  air  forming  the  "  motive  column,"  equivalent  to  the 
water  gauge.  To  obtain  the  power,  dividing  by  33,000,  as  in  other 
cases,  gives  the  actual  horse-power  expended  in  the  air. 

This  is  the  power  that  must  be  delivered  to  the  air.  But  in  all 
coal  mines,  and  in  some  other  mines,  the  upcast  shaft  has  a  certain 
ventilating  value.  The  air  from  the  workings  is  usually  at  a  higher 
temperature  than  that  which  enters  the  downcast  shaft ;  in  addition, 
it  is  largely  charged  with  moisture,  and  the  two  combined  make  a 
column  of  air  in  the  upcast  of  smaller  weight  than  that  in  the 
downcast,  the  difference  between  the  weight  of  the  two,  which  in 
deep  mines  may  be.  considerable,  being  called  the  "  motive  column." 
In  the  early  days  of  mining,  and  in  some  metalliferous  mines  at  the 
present  day,  the  motive  column  is  the  only  source  of  a  ventilating 
current,  and  it  drives  the  current  of  air  through  the  workings,  and  up 
the  upcast  pit.  With  furnace  ventilation,  the  motive  column  was 
created  by  heating  the  air  at  the  bottom  of  the  upcast  pit,  and 


DRIVING   MACHINES   BY   ELECTRICITY         341 

creating  a  column  of  air  at  higher  temperature  than  that  in  the 
downcast.  The  volume  of  the  motive  column  is  found  from  the 
following  table : — 

A"  water  gauge  represents  a  motive  column  of    32*2  feet 

1 "  64-4 

»>  »  »  j>  » 

2"  ,,  ,,  „  „  128*8    „ 

and  so  on. 

The  power  required  to  be  delivered  to  the  air  by  the  fan  will  be 
the  amount  of  power  required  to  be  delivered  to  the  air  less  that 
furnished  by  the  motive  column,  where  one  exists,  and  the  power 
that  must  be  delivered  to  the  fan  blades  is  this  net  power  equated 
with  the  efficiency  of  the  fan,  which  may  be  taken  at  from  44  per 
cent,  to  67  per  cent.,  so  that  the  power  to  be  delivered  to  the  fan 
pulley  is  the  net  power  multiplied  by  if$,  say,  taking  50  per  cent,  as 
an  average  efficiency. 

The  power  required  by  the  motor  driving  any  fan  is  again  the 
power  required  to  be  delivered  to  the  pulley  of  the  fan,  equated  with 
the  efficiency  of  the  motor.  If  we  take  the  efficiency  of  the  motor  as 
90  per  cent.,  and  that  of  the  fan  as  50  per  cent.,  the  net  power 
required  at  the  fan  blades  must  be  multiplied  by  ^j.0  for  the  power 
required  at  the  terminals  of  the  fan  motor. 


Driving  Air  Compressors 

Compressed  air  was  in  the  field,  for  the  transmission  of  power  in 
mines,  a  long  time  before  any  one  ventured  to  hope  that  electricity 
would  take  its  present  position.  Thirty  years  ago,  and  before,  com- 
pressed air  was  waging  a  fight  with  steam,  which  had  been  the 
method  previously  adopted  for  delivering  power  in  the  mine.  Steam 
is  objectionable,  because  of  the  losses  by  condensation  in  the  steam 
pipes,  and  because  of  the  dampness  which  it  sets  up  in  the  mine 
workings,  which  leads  to  other  troubles.  Compressed  air  was  a 
great  improvement  on  this,  but  compressed  air  is  very  wasteful,  as 
already  explained.  The  apparatus  employed  with  compressed  air 
consist  of  the  air  compressor,  driven  by  a  steam  engine,  or  by  water 
power,  or  by  gas  power,  the  pipe  line  connecting  the  compressor 
with  the  apparatus  that  is  to  use  the  compressed  air,  and  the 
engines  which  use  it.  There  are  three  sources  of  loss,  apart  from  the 
friction  of  the  driving  engine.  In  the  first  place,  when  air  is  com- 
pressed, the  act  of  compression  heats  the  air,  and  this  heat  is  always 
dissipated,  under  all  mining  conditions,  before  the  air  is  used  as  a 
motive  power,  and  consequently  the  energy  expended  in  heating  the 
air  is  lost.  Further,  when  the  air  is  heated  in  compression  it 


342  ELECTRICITY   IN   MINING 

expands,  and  a  smaller  quantity  of  air  is  compressed  at  each  stroke  of 
the  compressor,  so  that  more  work  has  to  be  done  by  the  compressor 
to  furnish  any  given  quantity  of  air  at  a  given  pressure  at  the  face. 
The  air  compressor  consists  of  one  or  two  cylinders,  similar  to  steam 
cylinders,  in  which  pistons,  similar  to  steam  pistons,  work  to  and 
fro,  drawing  in  air  on  one  stroke,  compressing  it,  and  delivering  it  to 
the  pipe  line,  or  the  receiver,  on  the  return  stroke.  In  modern  air 
compressors  it  is  found  economical  to  deprive  the  air  of  the  heat 
liberated  in  it  by  compression,  as  far  as  possible,  as  it  is  created.  And 
this  has  led  to  the  compressor  being  divided  into  two  and  sometimes 
more  cylinders.  The  air  is  compressed  to  a  certain  pressure  in  one 
cylinder,  and  is  forced  from  there  into  a  receiver,  where  it  is  cooled ; 
it  then  passes  into  a  second  compressor,  where  the  compression  is 
completed,  and  it  is  again  cooled,  and  is  then  delivered  to  a  receiver, 
usually  consisting  of  a  boiler  without  flues,  from  which  the  pipe  line 
leading  to  the  face  takes  its  supply.  In  some  forms  of  compressor, 
the  cylinders  are  surrounded  by  water,  a  portion  of  which  is  open  to 
the  atmosphere,  the  evaporation  from  the  exposed  surface  of  the  water 
tending  to  cool  the  cylinder.  In  other  forms  the  cylinders  are  fitted 
with  water  jackets,  similar  to  gas  engines,  and  cooling  water  is  kept 
circulating  through  them.  The  second  source  of  loss  is  in  the  pipe 
line,  and  it  is  made  up  of  two  portions,  the  loss  of  pressure  due  to  the 
friction  of  the  air  passing  through  the  pipes,  and  the  loss  of  air  itself, 
as  explained  in  a  previous  part  of  this  chapter,  owing  to  leakage  at 
the  joints  of  the  pipes.  The  loss  of  pressure  due  to  friction  is  usually 
very  small,  unless  the  plant  is  very  badly  designed ;  but  the  loss  due 
to  leakage  is  usually  very  great  indeed. 

The  third  source  of  loss  is  in  the  conversion  of  the  energy  stored  in 
the  compressed  air  into  mechanical  energy  in  the  motor.  It  is  usual 
to  employ  ordinary  engines  made  for  steam  to  use  the  compressed  air, 
and  here  two  sources  of  loss  arise.  One  that  for  some  time  created 
a  great  deal  of  trouble,  but  which  has  lately  been,  the  author  believes, 
practically  overcome,  was  the  freezing  of  the  moisture  contained  in 
the  air,  in  the  exhaust  ports  of  the  motor  cylinder.  It  is  usual  to 
take  the  air  for  the  compressor  on  the  surface  from  the  surrounding 
atmosphere,  which  is  always  more  or  less  charged  with  moisture,  with 
the  result  that  the  quantity  of  useful  air  is  less  than  it  would  be  if 
moisture  were  not  present,  by  the  cubical  content  of  the  moisture. 
This  moisture,  if  it  is  allowed  to  pass  on  into  the  pipe  line,  and  from 
the  pipe  line  to  the  motor  engine,  passes  from  the  state  of  vapour  to 
the  liquid  state,  and  freezes  in  the  exhaust  ports,  partially  or  wholly 
closing  them  up,  and  in  any  case  creating  considerable  back  pressure, 
lowering  the  efficiency  of  the  engine,  and  decreasing  the  amount  of 
work  it  is  able  to  do.  The  compressed  air  operates  in  the  motor 
engine  by  expanding,  just  as  steam  does  in  steam  cylinders,  and  as 


DRIVING   MACHINES   BY   ELECTRICITY         343 

the  air  in  the  cylinder  of  a  gas  engine  does  on  explosion.  But  in 
order  that  the  air  shall  be  enabled  to  expand,  it  must  absorb  heat, 
and  it  takes  this  from  surrounding  objects ;  the  walls  of  the  cylinder, 
etc.,  and  its  own  temperature  being  also  considerably  reduced,  it  can 
no  longer  support  the  moisture  it  has  carried  as  vapour,  the  moisture 
being  thereby  deposited,  and  heat  being  also  extracted  from  it,  as  well 
as  from  the  cylinder,  etc.,  the  moisture  is  converted  into  snow  and 
ice.  This  difficulty,  the  author  believes,  has  been  overcome  in  modern 
plant  by  allowing  the  moisture  to  drain  from  the  pipes  before  the 
air  arrives  at  the  motor  engine.  It  will  be  understood  that  when  the 
air  first  issues  from  the  compressor,  even  when  it  has  been  subject  to 
the  cooling  before  mentioned,  it  is  still  at  a  higher  temperature  than 
that  of  the  surrounding  atmosphere,  and  is  able  to  carry  a  compara- 
tively large  quantity  of  moisture  in  suspension.  The  receiver,  how- 
ever, which  is  employed  with  most  compressed  air  plants,  and  which 
is  usually  placed  in  the  open,  with  a  large  surface  exposed  to  the 
atmosphere,  cools  the  air  very  considerably,  and  a  large  proportion 
of  the  moisture  carried  by  the  air  is  condensed,  falls  to  the  bottom 
of  the  receiver,  and  is  drawn  off  in  the  usual  way.  As  the  air  passes 
through  the  pipe  line,  further  cooling  takes  place,  though  probably 
in  some  of  the  deep  mines  warming  may  take  place,  and  there  is 
usually  a  second  receiver  near  the  face,  where  any  moisture  is  allowed 
to  drain  out  of  the  air,  and  is  drawn  off.  In  the  author's  opinion,  it 
would  be  far  better,  and  more  economical,  so  far  as  this  part  of  the 
subject  is  concerned,  to  handle  the  air  on  its  way  to  the  compressor 
by  a  cooling  apparatus,  such  as  those  employed  in  connection  with 
the  cooling  and  drying  of  blast  furnace  air.  This  would  get  rid  of 
the  moisture  trouble,  and  it  would  also  raise  the  efficiency  of  com- 
pression by  enabling  a  larger  quantity  of  useful  air  to  be  taken  in  at 
each  suction  stroke  of  the  compressor.  The  other  portion  of  the  loss 
at  the  motor  engine  is  the  inability,  up  to  the  present,  so  far  as  the 
author  has  been  able  to  ascertain,  to  use  the  air  in  the  motor  cylinder 
expansively.  In  the  steam  engine,  it  will  be  remembered,  the  steam 
is  allowed  to  enter  at  boiler  pressure  for  only  a  short  portion  of  the 
stroke,  the  remainder  of  the  stroke  being  performed  by  the  expansion 
of  the  steam  itself,  and  the  efficiency  of  the  arrangement  being  thereby 
considerably  increased.  The  two  portions  of  work  done  by  steam  in 
a  steam  engine  may  be  compared,  the  first  portion  to  a  push  by  the 
steam  straight  from  the  boiler  to  the  piston,  and  the  second  to  the 
expansion  of  a  compressed  spring.  In  the  air  motor  engine  only 
the  first  portion  has  been  so  far  utilized.  The  air  enters  the  motor 
cylinder  from  the  receiver  or  the  pipe  line,  and  simply  pushes  the 
piston  to  the  end.  The  losses  in  compression  and  the  losses  in  the 
motor  cylinder  cannot  be  avoided  by  the  use  of  electricity,  except, 
perhaps,  the  latter  indirectly;  but  the  losses  in  the  pipe  line  may  be 


344  ELECTRICITY   IN   MINING 

reduced  by  compressing  the  air  near  the  point  of  consumption,  using  an 
electric  motor,  taking  its  power  from  the  supply  service  to  drive  the 
air  compressor,  the  compressed  air  being  used  to  drive  the  motor 
engine  as  before.  This  method  has  long  been  used  in  Germany,  and 
was  claimed  by  the  Germans  to  be  economical  a  good  many  years 
ago.  It  has  lately  been  introduced  into  this  country,  and  is  apparently 
making  way,  the  combined  efficiency  of  the  electric  drive  and  the 
compressed  air  being  claimed  to  be  60  per  cent,  at  the  compressed- 
air  receiver.  Fig.  156  shows  the  relative  efficiencies  of  a  compress- 
ing plant,  with  the  compressor  on  the  surface  and  in-bye;  and 
Plates  3lA  and  3lB  show  forms  of  air  compressors  electrically  driven  for 


voo 


•K) 


o 


A'  Indicated  Horse  Power  af  BanK 
B'ElecrTical        •  .... 

C«Elecrricai  Horse  Fbwer  ar  Compressor 
D"  Air  Morse  Power  at  Bank 
E* Face. 


MOE 


FIG.  156. — Diagram  furnished  by  the  Westinghouse  Co.,  showing  the  Efficiencies 
of  Compressors  driven  at  Bank  and  in-Bye. 

use  in-bye.  There  are  one  or  two  difficulties  in  connection  with  this 
arrangement.  In  the  deep  coal  mines  in  this  country  the  atmosphere 
near  the  face,  where  the  air  compressor  would  be  driven,  is  very  warm, 
a  temperature  of  80°  Fahr.  being  quite  common,  and  it  is  also  largely 
charged  with  moisture,  hence  the  quantity  of  air  taken  at  each 
suction  stroke  of  the  air  compressor  will  be  less  than  if  air  at  ordinary 
atmospheric  temperatures  in  this  country  could  be  employed.  On 
the  other  hand,  it  should  be  possible,  and  the  author  understands 
that  some  attempts  have  been  made,  to  adopt  the  plan  that  is 
employed  where  air  is  used  for  cold  storage  purposes,  and  to  use 
the  same  air  over  and  over  again,  adding  a  cooling  and  drying 
apparatus  to  the  plant.  Objections  to  this  are,  that  the  compressed- 


PLATE  28A.— Electrically  Driven  Winding  Plant,  by  the  Electrical  Co. 
fixed  in  a  German  Mine. 


PLATE  28s. — Electric  Winding  Motor  and  Brake,  as  fixed  by  the 
International  Electrical  Engineering  Co.  of  Liege,  at  the  Waihi 
Junction  Mine,  New  Zealand. 


[To  face  p.  344. 


DRIVING   MACHINES   BY   ELECTRICITY         345 

air  pipes  must  be  doubled,  but  as  they  need  not  be  large,  the  author 
believes  this  would  not  be  serious;  and  another  difficulty  is,  the 
neighbourhood  of  the  face  of  the  coal  is  not  a  convenient  spot  to 
employ  cooling  apparatus.  Still,  with  a  little  care  and  skill  in 
working,  it  might  be  done,  and  in  that  case  the  trouble  with  moisture 
should  be  completely  got  rid  of,  and  the  whole  efficiency  of  the  plant 
considerably  increased.  There  is  also  another  point  that  is  worth 
considering,  and  that  is  the  employment,  in  certain  cases  and  under 
certain  conditions,  of  an  electric  current  taken  from  the  power 
service  to  heat  the  air  before  it  enters  the  motor  cylinder.  As  in  all 
these  cases,  the  question  whether  such  a  course  would  be  economical 
or  not  can  only  be  determined  by  a  balance  sheet.  A  certain  definite 
number  of  heat  units  may  be  delivered,  say,  to  a  pipe  or  a  receiver, 
through  which  the  air  is  passing,  and  a  certain  definite  proportion  of 
that  heat  will  pass  into  the  air,  increasing  its  cubical  content,  and 
therefore  increasing  the  work  that  it  will  perform  in  the  motor  engine. 
This  current  will  cost  a  certain  amount  to  produce  in  the  generating 
station.  It  will  save  a  certain  amount  of  current  in  the  motor 
driving  the  air  compressor.  If  the  saving  is  sufficient,  or  if  convenience 
comes  in,  and  increases  its  value,  as  often  happens,  it  may  be  worth 
applying,  but  the  engineer  in  each  case  must  put  down  the  cost  on 
both  sides  and  compare  them.  In  the  very  early  days  of  electric 
lighting,  the  author  put  down  a  small  electric-lighting  plant  in  a 
colliery  nearly  half  a  mile  deep,  where  compressed  air  was  the  only 
means  of  driving,  and  to  get  over  the  trouble  of  the  moisture  freezing 
in  the  ports  of  the  driving  engine,  he  heated  the  air  passing  into  the 
engine  cylinder  by  means  of  the  current  that  supplied  the  lights.  The 
method  was  successful,  but  he  is  not  prepared  to  say  that  it  was 
economical. 


Power  required  for  Driving  Air  Compressors 
Underground 

The  power  required  to  drive  the  air  compressor  underground  is 
found  by  taking  the  power  required  to  compress  air  at  the  tempera- 
ture of  the  atmosphere  in  the  neighbourhood  of  the  compressor,  and 
with  the  moisture  that  is  usually  present  there,  from  the  atmospheric 
pressure  there,  up  to  60  Ibs.  or  80  Ibs.  per  square  inch,  whatever  may 
be  the  pressure  employed.  It  will  be  understood  that  each  cubic 
foot  of  air  requires  the  expenditure  of  a  certain  quantity  of  energy 
to  compress  it  from  the  atmospheric  pressure  to  60  Ibs.  or  80  Ibs.,  as 
the  case  may  be,  and  where  the  air  is  at  a  higher  temperature  than 
the  average,  taking  the  average  in  this  country  as  60°  P. — in 
America  it  is  taken  at  70°  F.,  and  in  tropical  countries  it  would  of 


346  ELECTRICITY   IN   MINING 

course  be  higher — more  energy  is  required  to  be  expended  than 
when  it  is  at  the  average  temperature,  and  with  the  average  quantity 
of  moisture.  Taking  the  average  temperature  where  the  air  com- 
pressor would  be  fixed  in  a  colliery  at  80°  F.,  each  cubic  foot  of  air 
would  require  from  0'5  to  07  H.P.  to  compress  and  deliver  it  at  GOlbs., 
and  from  0'76  to  1*0  H.P.  to  compress  and  deliver  it  at  80  Ibs.  So 
that  it  is  a  simple  calculation  to  find  what  horse-power  is  required  to 
compress  a  certain  number  of  cubic  feet  per  minute,  to  a  given  pressure. 
This  horse-power  is  to  be  delivered  to  the  piston  or  pistons  of  the  air 
compressor,  plus  the  power  required  to  overcome  the  friction  of  the 
compressor  itself,  and  the  power  required  to  be  delivered  at  the 
terminals  of  the  electric  motor  driving  the  compressor  will  be  found 
by  equating  the  efficiency  of  the  compressor  with  the  efficiency  of 
the  electric  motor,  as  explained  in  previous  cases. 


Electric  Locomotives 

The  electric  locomotive  has  not  been,  up  to  the  present,  employed 
in  mines  in  the  United  Kingdom,  but  it  is  largely  employed  both 
underground  and  on  the  surface  in  America  and  on  the  Continent,  and 
in  gold  mines  in  New  Zealand,  and  the  author  believes  would  do  good 
service  in  certain  cases  in  this  country.  The  mining  electric  locomo- 
tive is  a  small  copy  of  the  railway  electric  loco.  It  consists  of  a 
carriage,  of  the  form  shown  in  Plates  25B  and  26B,  mounted  on  four 
wheels  arranged  to  run  on  the  mine  waggon  tram  rails,  and  carrying  a 
motor,  usually  of  from  9  H.P.  upwards,  geared  to  one  axle  of  the  wheels. 
The  carriage  carries  a  seat  for  the  driver  behind,  a  head  light  in  front, 
a  short  trolley  pole,  which  is  again  very  similar  to  those  used  on  tram 
lines  and  railways,  but  smaller,  and  a  controller  of  a  similar  type  to 
those  employed  on  railways,  with  the  usual  brake,  both  at  the  driver's 
hand.  The  loco  takes  its  current  from  an  overhead  copper  wire, 
suspended  over  the  middle  of  the  track,  the  return  current  being  by 
means  of  the  rails,  the  current  passing  from  the  motor  to  the  wheels 
and  thence  to  the  rails.  The  pressure  usually  employed  with  mine 
locos  is  500  volts,  but  it  will  be,  of  course,  that  of  the  service. 
There  is  another  form  of  electric  locomotive  employed  in  Germany, 
of  which  one  was  exhibited  at  the  Glasgow  Exhibition,  in  which  a 
battery  of  accumulators  is  employed,  instead  of  the  trolley  pole  and 
trolley  wire.  This  is,  of  course,  a  much  safer  arrangement,  but  it  is 
doubtful  if  it  is  so  economical,  for  the  reasons  that  have  been 
explained  in  connection  with  accumulators. 

The  trolley  employed  in  America  with  mine  locomotives  is  nearly 
always  the  wheel  with  which  every  one  is  familiar  as  employed  on 
electric  tram  lines.  In  Germany,  however,  two  modifications  are 


DRIVING   MACHINES   BY   ELECTRICITY         347 

employed.  In  one  there  is  a  roller  about  two  inches  in  diameter,  of 
aluminium,  the  roller  extending  nearly  the  whole  width  of  the  track, 
and  being  supported  by  uprights  from  the  top  of  the  locomotive. 
This  has  the  advantage  that  it  is  not  easy  for  the  trolley  to  get  off 
the  trolley  wire,  and  any  wear  due  to  sparking  is  distributed  over  a 
very  much  larger  surface,  while  it  should  also  be  much  more  easy  for 
the  loco  to  go  round  a  curve,  than  with  the  wheel  trolley.  The  other 
arrangement,  which  is  coming  into  use  very  much  in  Germany  on 
electric  railways,  is  the  bow  trolley.  It  consists  of  a  rectangular  bow 
of  stout  wire,  held  in  the  position  usually  occupied  by  the  trolley 
pole  and  wheel,  and  it  rubs  against  the  underside  of  the  trolley  wire. 
It  has  the  advantage  that  it  is  not  necessary  when  reversing  to  turn 
the  trolley  pole  round;  the  bow  trolley  reverses  itself  when  the 
locomotive  has  run  a  certain  distance  back. 

For  surface  work  in  mines  in  the  United  Kingdom  and  elsewhere, 
the  author's  view  is  that  electric  locos  would  be  of  great  service,  and 
there  should  be  no  difficulty  whatever  in  arranging  overhead  trolley 
wires.  The  locomotives  can  be  small,  for  hauling  mine  waggons,  or 
they  can  be  large  enough  to  handle  railway  waggons.  For  under- 
ground work,  however,  it  is  very  doubtful  whether,  except  on  the 
main  intake  roads,  it  would  be  wise  to  employ  the  electric  locomotive. 
Its  use  necessitates  a  bare  conductor  overhead,  the  pressure  of  the 
service  being  present  at  every  point  of  the  conductor,  and  there 
being  the  consequent  danger  of  workmen  and  others  getting  shocks 
from  it.  In  some  mines  in  America,  the  difficulty  has  been  over- 
come by  enclosing  the  trolley  wire  inside  an  inverted  wooden  trough. 
This  should  protect  miners  from  shock,  as  long  as  the  trough  is  perfect, 
but  the  author  would  be  afraid  that  the  trough  itself,  becoming  wet, 
would  set  up  considerable  leakage  on  the  trolley  wire. 


The  Electric  Driving  of  other  Machinery  about 

the  Mine 

In  the  foregoing  pages  the  author  has  described  the  principal 
machines  to  which  electrical  driving  has  been  applied,  mainly  because 
of  the  efficiency  of  the  electrical  method  of  transmitting  power  to  a 
distance,  but  when  once  electrical  power  is  on  the  ground,  it  may  be 
employed,  and  usually  economically,  for  driving  any  and  every 
machine  about  the  place.  About  the  surface  of  every  mine,  coal  or 
metalliferous,  there  are  nearly  always  isolated  engines  taking  steam 
from  the  boilers  through  pipes  that  are  laid  in  the  ground,  or  some- 
times carried  overhead,  and  there  are  always  considerable  losses  from 
condensation  of  steam  in  these  pipe  lines,  and  in  addition,  when  the 
engine  is  to  be  started,  the  condensed  water  must  be  got  rid  of,  the 


348  ELECTRICITY  IN   MINING 

engine  must  be  very  carefully  started,  or  it  will  knock  itself  to  pieces, 
and  this  means  time.  With  an  electrical  power  service,  cables  may 
be  led  overhead,  or  in  the  ground,  to  the  neighbourhood  of  every 
machine  that  is  to  be  driven,  a  motor  may  be  fixed  to  take  the  place 
of  the  engine  that  was  previously  driving,  in  any  convenient  manner, 
either  by  belt,  worm  or  spur  gearing,  or  by  direct  connection,  and  the 
motor  will  use  nothing,  so  long  as  the  cables,  switches,  etc.,  are  kept 
in  order,  except  when  it  is  doing  useful  work,  and  it  will  always  be 
ready  to  start  quickly,  say  in  a  few  seconds.  This  applies  to  coal- 
washing  machines,  which  may  be,  and  are,  driven  by  electric  motors, 
fitting-shop  shafting,  cranes,  surface  haulage,  creepers,  pumps  for 
boiler  feed  at  a  distance  from  the  mine,  stamps  for  metalliferous 
mines,  or  breakers,  sizers,  etc.  The  most  convenient  forms  of  motors 
employed,  if  the  service  is  continuous  current,  will  usually  be  the 
shunt-wound  motor,  with  a  series  coil  for  starting,  if  the  machine  is 
to  start  against  a  heavy  load.  Three-phase  motors  will  also  be 
suitable,  and  can  be  of  the  squirrel-cage  type  for  small  work,  or 
where  the  machine  can  drive  on  to  a  fast  and  loose  pulley ;  wound 
rotors  being  employed  for  heavier  work,  where  the  motor  has  to  start 
against  a  load.  Electric  motors  are  arranged  with  spur-reducing 
gear  attached,  ready  for  fixing,  to  drive  any  machine  that  may  be 
required.  There  may  also  be  apparatus  underground,  besides  those 
mentioned,  that  could  be  conveniently  driven  by  electric  motors. 
Plates  31c  and  3lD  show  Blackett's  coal  conveyer,  for  taking  the 
coal  from  the  face,  in  the  seams,  and  delivering  it  to  mine  waggons 
at  the  gate-end  road.  It  is  shown  driven  by  an  electric  motor. 

For  each  motor  there  should  be  a  small  switchboard,  preferably 
of  enamelled  slate,  fixed  to  steel  supports,  carrying  the  starting 
switch  and  resistance,  the  no-load  and  overload  circuit  breakers, 
and  an  emergency  switch,  the  whole  being  enclosed  in  a  lock-up 
cupboard. 

Estimating:  the  Power  given  out  by  Steam  or 
Compressed-air  Engines  that  are  to  be  dis= 
placed  by  Electric  Motors 

It  has  been  explained  that  it  is  wise  when  estimating  the  power 
required  in  any  electric  motor  to  drive  any  given  machinery,  to  take 
the  power  given  out  by  the  steam  or  compressed-air  engine  the 
motor  is  to  displace.  It  is  sometimes  not  easy  to  estimate  this.  The 
safest  plan  is,  of  course,  to  indicate  the  engine,  and  then  it  is  a  simple 
calculation  from  the  well-known  formula.  But  it  will  be  often  very 
inconvenient  to  indicate  the  engine.  The  engine  driving  a  screening 
plant,  or  a  battery  of  stamps,  or  a  sorting  plant,  often  cannot  easily 


DRIVING   MACHINES   BY   ELECTRICITY         349 

be  got  at  to  indicate,  and  very  frequently  it  must  not  be  stopped  for 
a  sufficient  time  to  allow  of  the  measurements  being  taken.  Further, 
in  a  very  great  many  cases  the  engines  employed  for  this  work  do 
not  work  expansively,  at  least  not  beyond  the  ordinary  expansion 
provided  by  the  arrangement  of  the  valves,  as  the  engine  leaves  the 
makers.  In  the  great  majority  of  cases,  economy  of  steam  is  not 
sought  for,  where  driving  of  machines  is  concerned,  unless  the  engine 
is  driving  a  number  of  machines  through  shafting,  and  it  is  con- 
venient to  make  the  necessary  arrangements  for  working  it  expan- 
sively. What  is  usually  wanted  is  that  the  machine  shall  keep  on 
working  during  the  full  working  day,  and  for  that  purpose  the  engine 
is  required  to  have  plenty  of  power,  and  further,  it  is  often  not 
convenient  for  it  to  be  of  the  larger  size  necessary,  if  it  is  to  work 
expansively.  Hence,  engines  for  this  purpose  are  frequently  sent  out 
with  their  valves  arranged  to  cut  off  at  half,  or  three-quarter  stroke, 
or  even  not  to  cut  off  at  all,  and  the  work  is  done  more  or  less  by  a 
push  from  the  steam,  coming  direct  from  the  boiler.  After  all,  the 
losses  by  condensation  are  far  more  than  any  losses  due  to  any 
working  expansively  in  small  engines  of  this  kind.  For  the  purpose 
of  estimating  the  power  that  an  electric  motor  to  take  the  place 
of  one  of  these  engines  should  furnish,  it  is  a  pretty  safe  rule  to  take 
the  size  of  the  cylinder,  the  length  of  the  stroke,  and  the  pressure 
of  the  steam  at  the  stop  valve,  applying  the  usual  engine  formula. 
If  this  rule  dictates  an  electric  motor  larger  than  could  possibly 
have  done  the  work,  it  will  be  a  good  fault.  The  tendency  when 
fitting  electric  motors  up  is  too  often  in  the  opposite  direction,  and 
this  leads  too  often  to  breakdowns  that  might  have  been  avoided, 
if  a  little  more  power  had  been  given. 


CHAPTER  VII 
FAULTS  IN  ELECTRICAL  APPARATUS 

CAUSES  of  failure  in  electrical  apparatus  are  known  as  "  faults."  A 
signal  bell  does  not  ring,  a  telephone  does  not  speak,  a  lamp  does  not 
burn,  a  motor  does  not  run,  or  any  one  of  these  fails  in  a  minor 
degree.  The  cause  is  what  is  known  as  a  "  fault."  All  faults  in 
electrical  mining  apparatus  are  due  to  one  of  two  causes,  frequently 
to  both. 

1.  The  interposition  of  resistance  in  the  conductive  path. 

2.  The  lowering  of  the  insulation  resistance  of  some  part  of  the 
apparatus. 

The  interposition  of  resistance  in  the  conducting  path  leads 
directly  to  apparatus  not  working  as  it  should  do.  A  bell  may  ring 
less  loudly,  or  not  at  all,  a  lamp  may  not  give  its  full  light,  or  may 
give  no  light,  and  so  on.  The  extreme  case  is,  where  there  is  a  break 
in  the  conducting  path,  as  when  the  conductor  itself  is  severed,  or 
when  some  part  of  the  apparatus,  some  two  surfaces  of  which  ought 
to  be  in  contact,  are  completely  separated. 

The  lowering  of  the  insulation  resistance  of  any  part  of  the 
apparatus,  such  as  the  insulation  of  the  generator,  the  cables,  the 
switch  gear,  leads  to  current  passing  through  the  insulation,  which 
ought  not,  and  this  leads  to  the  gradual  destruction  of  the  insulation, 
the  further  lowering  of  the  insulation  resistance,  and  in  addition  it 
lowers  the  pressure  beyond  the  point  at  which  the  leak  occurs.  Thus 
defective  insulation  of  some  part  of  the  generator  will  lower  the 
pressure  at  the  terminals.  Defective  insulation  in  a  pair  of  cables, 
say  in  a  mine  shaft,  lowers  the  pressure  at  the  pit  bottom,  and  so  on. 
The  extreme  case  of  lowered  insulation  is  what  is  known  as  a  "  short 
circuit,"  or  more  frequently  a  "  short,"  where  there  is  a  direct  con- 
nection, of  very  low  resistance,  between  two  points  in  the  circuit, 
between  which  the  pressure  of  the  service  exists.  Lowered  insulation 
resistance  often  leads  to  severance  of  the  conductor,  because  it  means 
that  the  insulating  envelope  has  been  damaged  by  water  or  in  some 


FAULTS   IN   ELECTRICAL   APPARATUS  351 

other  way,  and  the  water  having  passed  through  the  envelope  to  the 
conductor,  gradually  eats  the  latter  away,  interposing  resistance  as 
the  chemical  action  proceeds,  and  finally  severs  it.  Severance  of  the 
conductor  also  sometimes  leads  to  the  destruction  of  the  insulation 
in  the  neighbourhood  of  the  severance,  because  when  a  conductor 
through  which  a  current  is  passing,  under  considerable  pressure,  is 
parted,  a  spark  passes  across  the  break,  and  in  some  cases  an  arc  is 
formed  between  the  severed  ends  for  a  short  time,  the  arc  destroying 
the  insulation  in  its  neighbourhood  and  for  a  considerable  distance 
on  each  side. 


Rules  for  Testing 

There  are  a  few  simple  rules  that  are  applicable  to  all  kinds  of 
electrical  apparatus  in  testing  for  faults. 

1.  Always  make  sure  that  the  source  of  electricity,  the  battery  or 
the  dynamo,  is  doing  its  work  properly,  and  test  it  if  necessary  before 
proceeding  to  make  other  tests,  unless,  from  other  indications,  it  is 
known  that  the  fault  exists  in  some  part  of  the  apparatus  away  from 
the  source. 

2.  Always  work  outwards  from  the  generator  in  testing  for  faults, 
unless  there  are  indications  that  the  fault  is  in  a  certain  apparatus  at 
a  distance  from  the  generator. 

3.  A  great  deal  of  time  in  testing  for  "  faults  "  will  be  saved  if 
continuous  tests  are  made  upon  the  apparatus.     If  the  insulation 
resistance  of  each  part  of  the  apparatus  is  tested  periodically,  and 
recorded  in  a  book  provided  for  the  purpose,  it  will  be  seen  if  any 
portion  is  deteriorating,  and  when  this  is  shown,  the  earliest  opportu- 
nity should  be  taken  of  making  a  further  complete  test  of  that  part  of 
the  apparatus.     The  "  test  in  time  "  will  save  whoever  is  responsible 
for  it  a  great  deal  of  labour,  and  troublesome  labour,  that  he  will  have 
to  undertake  if  the  apparatus  is  allowed  to  break  down.     In  nearly 
every  instance,  signs  are  given  at  a  comparatively  early  date  that  will 
enable  a  careful  man  to  prevent  faults  occurring. 

4.  There  is  a  simple  rule  in  connection  with  all  faults,  and  it  is 
when  testing  and  you  come  to  two  points,  at  one  of  which  you  have 
indications  of  the  normal  conditions,  or  not  far  from  the  normal  con- 
ditions, and   at  the   other  you  have  indications  far  removed  from 
the  normal,  the  fault  is  almost  sure  to  lie  between  the  two.     Put  in 
another  form,  when  testing,  and  you  have  found  your  current  and 
lost  it  again,  the  fault  lies  between  the  point  at  which  you  last  found 
your  current  or  pressure,  and  the  point  at  which  you  lost  it.     With 
lighting  and  power  apparatus  a  good  deal  of  the  testing  will  be  made 
for  pressure,  and  this  rule  will  mean  that  when  you  suddenly  come  to 


352  ELECTRICITY   IN   MINING 

a  very  large  fall  of  pressure,  without  the  presence  of  any  apparatus 
taking  a  large  current  to  account  for  it,  you  will  have  just  pre- 
viously passed  over  the  fault. 

5.  In  testing  for  insulation  resistance  always  use  a  pressure  at 
least  as  high  as  that  of  the  service.     The  strain  upon  the  insulation 
increases  with  the  pressure,  and  one  will  often  obtain  a  false  indi- 
cation, showing  an  apparently  good  insulation  resistance,  with  a  low- 
pressure  current,  when  with  a  high-pressure  current  the  insulation 
resistance  might  be  completely  broken  down. 

6.  In  testing  for  continuity  of  conducting  path,  always  use  as  low 
a  pressure  as  possible,  and  for  practically  the  same  reason  as  you  use  a 
high  pressure  in  testing  for  insulation.     A  high  pressure  will  drive 
a  current  sometimes  through  a  comparatively  high  resistance,  where  a 
low  pressure  would  declare  the  existence  of  the  high  resistance. 

Faults  in  Mine  Signals 

Always  first  test  the  battery  which  supplies  current  to  the  bell 
that  does  not  ring.  If  the  battery  is  tested  frequently,  it  will  soon 
be  seen,  on  testing  each  cell,  if  any  one  or  two  cells  have  so  far  worked 
down  as  to  interpose  a  high  resistance  into  the  circuit.  The  battery 
may  be  tested  by  a  lineman's  galvanometer,  which  consists  of  a  pair 
of  coils  of  wire  surrounding  a  vertical  magnetic  needle,  the  needle 
boing  connected  to  a  vertical  pointer  which  moves  over  a  semi- 
circular dial,  graduated  on  each  side  from  0°  to  90°.  The  two  coils 
contain,  one  a  few  turns  of  comparatively  thick  wire,  and  the  other  a 
large  number  of  turns  of  very  thin  wire.  One  end  of  each  coil  is 
joined  to  one  terminal,  and  the  other  end  of  each  coil  to  a  separate 
terminal,  so  that  there  are  three  terminals  on  top  of  the  case  enclosing 
the  apparatus.  The  thick  wire  coil  is  used  for  testing  the  condition 
of  individual  cells,  the  thin  wire  for  testing  for  leakage  currents,  and 
sometimes  for  continuity  of  the  circuit.  One  form  of  lineman's 
galvanometer  is  shown  in  Fig.  157.  The  battery  may  also  be  tested, 
and  perhaps  more  conveniently  under  modern  conditions,  by  low- 
reading  voltmeters.  Voltmeters  are  made  now  to  read  to  five  and 
six  volts,  in  the  form  of  an  apparatus  that  can  be  carried  in  the 
waistcoat  pocket.  The  open  type  Le  Clanche,  the  dry  cell,  and  the 
bichromate  cells  will  all  give  about  1*3  volts  when  in  proper  working 
order,  when  the  voltmeter  wires  are  connected  to  the  terminals  of  the 
cell,  and  this  pressure  will  decrease  gradually  as  the  cell  is  used. 
When  the  cell  shows  only  about  O5  volts,  in  the  case  of  a  dry  cell,  it 
should  be  taken  out  of  the  battery  and  allowed  to  rest.  Sometimes 
a  cell  after  resting  will  recover,  and  will  continue  to  work  for  some 
little  time.  In  the  case  of  a  wet  cell,  it  may  sometimes  be  recupe- 
rated by  adding  sal-ammoniac,  by  cleaning  the  zinc,  or  adding  a 


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FAULTS   IN   ELECTRICAL    APPARATUS 


353 


fresh  zinc,  or  if  these  do  not  suffice,  by  changing  the  filled  porous 
cells.  The  filled  porous  cell,  again,  if  allowed  to  rest,  will  sometimes 
recover  itself,  and  may  be  used  again.  The  mercury  bichromate  cell 
may  be  recovered,  sometimes  by  adding  bichromate  of  potash,  some- 
times by  adding  a  little  sulphuric  acid,  sometimes  by  cleaning  the 
zinc,  and  by  scraping  off  the  crystals  that  are  formed  on  the  carbon 
plate.  The  condition  of  this  cell  is  known  very  accurately  by  the 
colour  of  the  solution.  When  in  good  working  order  it  is  a  bright 
orange,  it  gradually  becomes  paler,  then  green,  then  dark  blue. 
Occasionally  a  cell  of  this  type 
will  give  very  little  current,  even 
when  its  solution  is  a  bright 
orange;  it  then  requires  a  little 
sulphuric  acid. 

Do  not  attempt  to  recuperate 
either  dry  cells  or  any  other 
primary  battery  cells  by  means 
of  an  electric  current.  Text-book 
theory  states  that  this  may  be 
done.  Practical  theory  says  that 
it  cannot,  because  a  number  of 
other  chemical  actions  have  gone 
on  within  the  cell,  which  cause 
any  recuperation  that  may  be 
attained  to  be  only  temporary. 
In  particular,  the  carbon  plate  is 
almost  always  attacked,  in  all 
forms  of  cells,  by  the  secondary 
salts  that  are  formed  in  the  work- 
ing of  the  battery,  arid  they  are 
not  dislodged,  and  the  carbon  plate 
is  not  restored,  by  putting  a  reverse 
current  through  it.  In  any  case, 
it  is  far  cheaper  to  change  the  cells 
when  necessary  than  to  tinker 
with  recuperation. 

Having  made  sure  the  battery  is  right,  examine  the  bell  for  dis- 
connection. In  testing  signals  and  telephones,  and  for  a  good  deal  of 
electric  light  and  power  work,  a  dry  cell  and  either  a  lineman's 
galvanometer,  or  a  low-reading  ampere  meter,  will  be  found  very 
useful.  Wherever  a  disconnection  is  suspected  a  circuit  is  easily 
formed  of  the  dry  cell,  the  instrument  and  the  apparatus,  and  the 
disconnection  either  found  or  its  presence  shown  to  be  non-existent  in 
a  few  minutes.  A  somewhat  frequent  source  of  trouble  with  bells  is, 
the  ends  of  the  wire  coils  break  off  where  they  leave  the  coil,  or 

2  A 


FIG.  157. — Lineman's  Galvanometer.  Two 
of  the  Terminals  are  connected  to  the 
Low  Resistance  Thick  Wire  Coils  and 
two  to  the  High  Eesistance  Fine  Wire 
Coils,  one  Terminal  being  common  to 
both  Coils.  It  will  be  noticed  that  it 
has  a  Leather  Strap  for  carrying. 


354  ELECTRICITY   IN   MINING 

where  they  are  connected  to  a  terminal,  and  the  wire  is  sometimes 
held  together  by  the  cotton  or  silk  covering.     A  test  will  show  the 

Eresence  of  this,  and  a  little  gentle  pulling  will  disclose  the  actual 
mlt. 

Another  somewhat  frequent  source  of  trouble  with  single-stroke 
bells  is,  the  armature  sometimes  remains  in  front  of  the  electro- 
magnet after  a  signal  has  been  given,  in  place  of  falling  baok.  This 
may  be  due  to  a  leakage  current,  or  it  may  be  due  to  imperfect  con- 
struction of  the  bell.  In  either  case,  there  should  be  a  provision  for 
throwing  the  armature  away  from  the  magnet  by  means  of  either  a 
spiral  or  straight  spring,  as  convenient.  The  centi-ampere  meter,  so 
strongly  recommended  in  connection  with  mine  signals,  will  save  a 
great  deal  of  trouble  and  time  in  testing.  It  will  show,  as  explained, 
if  there  is  a  leakage  current  on,  and  it  will  be  a  simple  matter  to 
increase  the  tension  of  the  throw-off  spring  of  the  bell,  when 
necessary,  to  keep  the  armature  clear,  and  keep  the  bell  working  till 
the  leakage  is  got  rid  of. 

To  test  for  a  disconnection  either  in  the  covered  wires,  say  in  the 
shaft,  or  the  iron  wires  on  the  engine  road,  there  is  one  simple  rule. 
Taking  the  engine  road  first — a  frequent  source  of  trouble  is,  coal 
dust  accumulates  on  the  wires,  and  prevents  proper  contact  being 
made.  Take  a  voltmeter  arranged  to  read  the  full  pressure  of  the 
battery,  and  test  at  the  commencement  of  the  engine-road  wires,  and 
then  at  different  points  on  the  road,  proceeding  outwards,  first  with 
the  wires  as  they  are,  and  secondly,  after  scraping  them.  There  will 
be  a  gradual  fall  of  pressure  as  the  distance  from  the  engine  house 
increases,  and  if  there  is  a  sudden  serious  fall  between  any  two  points 
the  fault  will  be  found  between  them.  A  common  fault  in  this  case 
is  a  badly  made  joint  in  the  iron  wire.  In  jointing  iron  wires,  the 
ends  should  be  scraped  very  clean,  and  they  should  either  be  bound 
tightly  together  with  iron  wire  that  has  also  been  scraped  clean,  or 
they  should  be  twisted  firmly  together  with  what  is  known  as  a  bell- 
hanger's  twist.  The  fault  may  also  occur  in  the  covered  wire  between 
two  pieces  of  road,  where  the  iron  wires  are  terminated  on  each  side 
of  the  junction,  and  covered  wire  employed  to  connect  them.  The 
difference  of  pressure  on  the  two  sides  of  the  junction  will  show  this. 
Where  the  disconnection  occurs  in  the  shaft  wire,  careful  test  should 
be  made  at  the  top  of  the  shaft  and  at  the  bottom  for  pressure 
between  the  wire  carrying  the  current  to  the  bell  and  the  return  wire. 
The  existence  of  a  large  difference  of  pressure  between  the  top  and 
bottom  of  the  shaft  will  show  that  the  trouble  is  in  the  shaft,  and  the 
shaft  wires  should  then  be  very  carefully  examined,  every  inch  of  the 
wire  being  passed  through  the  hand,  and  as  good  a  light  as  can  be 
obtained  used.  A  disconnection  will  nearly  always  be  shown,  by  the 
presence  of  the  green  salt  of  copper,  that  is  formed  by  the  chemical 


FAULTS   IN   ELECTRICAL   APPARATUS  355 

action  between  the  pit  water  and  the  copper  wire.  It  requires, 
however,  some  practice  in  looking  for  faults  of  this  kind  to  discover 
it,  and  any  man  in  charge  of  electric  signals  who  examines  shaft  wires 
for  the  first  time  should  go  carefully  through  them  several  times 
before  proceeding  to  the  next  step.  The  next  step  unfortunately  is 
one  that  is  unavoidable  in  mine  signals,  and  it  is  one  which  will 
probably  lay  the  seeds  of  trouble  in  the  future.  Having  made  sure 
there  is  a  disconnection  in  the  shaft,  a  test  must  be  made  about  the 
middle  of  the  shaft,  by  removing  the  covering  from  the  two  wires 
forming  the  circuit,  and  taking  the  pressure  across  them.  If  the 
pressure  that  should  exist  at  that  point — a  little  less  than  that  on 
either  the  pit  top  or  pit  bottom,  according  as  it  is  a  down  or  up 
signal — exists,  the  fault  is  either  below  or  above,  according  as  it  is 
down  or  up  signal,  and  the  process  must  be  repeated,  say,  halfway 
between  the  middle  of  the  shaft  and  the  bottom,  again  halfway 
between  that  and  the  bottom  of  the  shaft,  and  so  on,  until  the  section 
is  found  in  which  the  fault  exists.  The  places  where  tests  are  made 
should  be  wiped  as  dry  as  possible,  immediately  the  test  is  complete, 
and  they  should  also  be  immediately  wrapped  with  two  or  three 
coatings  of  strip  rubber,  two  or  three  coatings  of  primed  tape  being 
wrapped  over  them,  and  the  whole  protected  by  yarn,  and  so  on. 
These  points  where  tests  have  been  made  should  be  examined  as  fre- 
quently as  possible,  as  no  matter  how  carefully  the  recovering  is  done, 
water  will  almost  surely  penetrate,  and  fresh  faults  will  be  made. 
Leakage  on  a  mine  signal  is  shown  by  a  battery  working  down  very 
rapidly,  and  bells  refusing  to  work  beyond  a  certain  point.  The  test 
for  leakage  on  mine  signals  is,  to  break  the  signal  in  parts  at  con- 
venient points,  as  say  the  pit  bottom,  the  junctions,  and  so  on,  and 
either  observe  the  effect  upon  the  centi-ampere  meter,  or  if  no  centi- 
ampere  meter  is  fixed  in  the  engine  house,  the  effect  upon  an  instru- 
ment, which  may  be  a  low-reading  ampere  meter,  or  the  fine  wire 
circuit  of  a  lineman's  galvanometer.  If  the  leakage  is  due  to  one 
particular  section,  as  sometimes  happens,  the  fact  will  be  shown  by 
the  disappearance  of  the  deflection  when  that  section  is  taken  off,  but 
if,  as  more  frequently  happens,  the  leakage  is  more  or  less  continuous 
all  the  way  through  the  signal,  owing  to  the  insulation  having 
generally  deteriorated,  the  fact  will  be  shown  by  the  deflection  on  the 
leakage  instrument  lessening,  as  successive  sections  from  the  end  are 
disconnected. 

Faults  in  Telephones 

Faults  in  telephones  are  of  two  kinds,  failure  of  the  calling 
apparatus,  and  failure  of  the  speaking  apparatus.  The  calling  appa- 
ratus for  private  telephone  services  is  nearly  always  now  the  magneto 


356 


ELECTRICITY   IN   MINING 


generator  and  the  magneto  trembling  bell.  The  magneto  generator 
will  show  if  it  is  in  order  by  ringing  its  own  bell,  or  the  reverse.  In 
any  case,  if  either  the  magneto  generator  or  magneto  bell  is  out  of 
order,  it  is  a  case  for  a  skilled  mechanic. 

Failure  of  the  speaking  portion  may  be  due  either  to  failure  of 
the  microphone  portion,  the  receiver  portion,  or  the  switch  gear.  The 
microphone  battery  is  a  frequent  source  of  trouble,  and  should  be 
tested,  as  explained  for  signal  batteries,  whenever  failure  of  speech 
occurs.  The  failure  of  the  microphone  battery  will  cause  the  sender's 
speech  not*  to  be  heard  by  the  receiver.  Another  fruitful  cause  of 
failure  in  telephones,  not  so  common  now  as  in  the  early  days,  is  the 


INDUCTION  ODIU 


FAULT 


SWITCH 


MICROPHONE 


D 


EARTH. 


BATTETRY 


EARTH 


FIG.  158. — Diagram  of  Connections  for  testing  a  Telephone  Set. 

switch.  The  hook  switch  is  arranged  to  put  the  calling  apparatus  in 
circuit  when  the  telephone  is  on  the  hook,  and  the  speaking  apparatus 
when  the  telephone  is  off  the  hook,  the  connections  being  made  by  a 
lever  sliding  between  springs.  These  springs  sometimes  work  loose, 
and  do  not  make  proper  contact,  the  result  being  sometimes  failure 
of  speech,  sometimes  intermittent  speech.  Another  cause  of  failure 
is,  the  small  wires  that  are  employed  in  the  small  induction  coil, 
fixed  in  the  telephone  transmitter,  may  be  broken  at  the  coil,  or  at 
one  of  the  screws  where  they  are  connected.  Either  of  these  breaks 
would  cause  speech  to  be  broken  completely.  All  of  these  things 
may  be  tested  with  the  dry  cell  and  the  lineman's  galvanometer,  or 


FAULTS   IN   ELECTRICAL   APPARATUS  357 

low-reading  ampere  meter,  making  up  the  different  circuits  through 
which  current  should  pass  with  the  dry  cell  and  the  instrument  in 
the  circuit,  and  testing  from  point  to  point  within  the  apparatus,  till 
two  points  are  reached,  at  one  of  which  a  deflection  is  shown,  and  at 
the  next  no  deflection.  The  break  lies  between  these  two.  The 
microphone  battery,  or  the  ringing  battery,  where  one  is  employed, 
may  be  used  for  testing  for  a  disconnection.  Fig.  158  shows  the 
connections  for  testing,  using  the  ringing  battery. 

It  is  not  often  that  leakage  interferes  with  the  working  of 
telephone  apparatus,  because  the  currents  are  so  small,  and  the 
pressures  are  also  so  small. 

Faults  in  Dynamo  Machines 

Dynamo  machines,  both  generators  and  motors,  are  subject  to  the 
two  faults  mentioned,  lowering  of  the  insulation  resistance,  and 
increase  of  conductivity  resistance.  If  the  insulation  resistance  of 
the  field  coils  is  lowered,  and  particularly  if  the  insulation  of  one 
coil  disappears  completely,  that  is  to  say,  if  the  coil  is  short  circuited, 
the  pressure  generated  by  the  machine  will  usually  be  lowered,  and 
the  other  field  coils  will  show  an  increase  of  temperature  above  that 
usual  under  ordinary  working  conditions.  The  rule,  therefore,  is,  in 
a  case  of  this  kind,  to  look  for  the  coil  which  is  cool,  and  test  its 
conductivity  and  insulation  resistance.  If,  as  sometimes  happens, 
the  insulation  between  the  inner  wire  and  the  outer  wire  has  broken 
down,  and  the  coil  is  practically  short  circuited,  this  will  be  shown 
immediately  by  the  conductivity  resistance  test.  Each  field  magnet 
coil  has  a  certain  resistance,  which  the  electrician  in  charge  of  the 
apparatus  should  know,  and  should  obtain  from  the  manufacturers 
when  the  apparatus  is  first  put  into  service,  and  if,  having  discon- 
nected this  coil  and  tested  it,  as  will  be  explained,  he  finds  the 
resistance  is  very  much  lower  than  it  should  be,  the  cause  is 
probably  the  insulation  has  broken  down. 

The  above  applies  equally  to  the  field  coils  of  continuous-current 
or  alternating-current  machines. 


Faults  in  Continuous-current  Armatures 

The  principal  points  where  faults  occur  in  continuous-current 
armatures  are,  at  the  commutator,  and  between  the  coils  and  the  iron 
core.  As  explained  in  Chapter  IV.,  the  two  ends  of  adjacent  coils 
are  brought  to  the  radial  bar  of  a  section  of  the  commutator,  and  are 
there  sometimes  merely  soldered,  sometimes  screwed  and  soldered. 
If  the  soldering  has  not  been  very  carefully  done,  if  the  slot  in  the 


358  ELECTRICITY   IN   MINING 

radial  bar  has  not  been  thoroughly  cleaned  and  thoroughly  tinned, 
and  the  ends  of  the  coils  also  thoroughly  cleaned  and  thoroughly 
tinned,  and  when  the  two  are  married,  the  whole  thoroughly  filled 
with  solder,  and  kept  hot  until  the  whole  has  become  one  solid  mass, 
and  then  allowed  to  cool  so  as  to  form  a  solid  mass,  the  vibration  of 
the  machine  will  sometimes  gradually  break  the  wires  away  from  the 
commutator,  and  a  very  troublesome  fault  indeed  is  set  up.  When 
a  break  occurs  at  the  commutator,  the  machine  refuses  to  give  any 
current.  A  small  spark  is  seen  at  the  commutator,  but  no  pressure, 
or  very  little,  appears  at  the  terminals  of  the  machine.  When  the 
fault  is  due  to  the  wires  coming  away  from  the  commutator  bar,  if 
the  machine  is  stopped  and  the  commutator  examined,  it  is  often 
very  difficult  indeed  to  find  the  faulty  one,  because  the  wires  will 
partly  go  back  into  their  place.  The  rule  is,  note  the  point  where 
most  sparking  occurs  when  the  machine  is  running,  and  examine  the 
commutator  connections  in  its  immediate  neighbourhood.  Failing 
this,  it  is  necessary  to  try  to  break  the  connections  one  by  one,  or  to 
endeavour  to  force  the  wires  out  of  their  slots.  The  wire  that  is 
loose  will  then  usually  come  out  with  comparative  ease.  Great  care, 
however,  is  necessary  in  doing  this.  The  other  principal  sources  of 
trouble,  the  connection  between  the  coils  and  the  core,  and  the 
connection  between  adjacent  commutator  segments,  or  between 
commutator  segments  and  the  axle,  are  due  to  the  insulation  of  the 
coils  and  of  the  commutator  segments,  and  of  the  commutator  from 
the  axle  being  broken  down.  In  modern  continuous-current  generators 
and  motors  there  is  often  a  considerable  difference  of  pressure 
between  adjacent  sections  of  the  commutator,  and  there  may  be 
between  the  coils  and  the  core,  and  between  the  commutator  and  the 
axle.  In  the  modern  dynamo,  this  is  fully  provided  for  by  special 
care  in  the  insulation,  as  described  in  Chapter  IV.  But  it  may 
happen  that  defective  mica  has  been  used.  There  are  different 
qualities  of  mica,  some  of  which  will  not  stand  high  pressure. 
Also,  with  very  high  pressures,  it  has  happened  that  nitric  acid  has 
been  generated  in  the  slots  in  which  the  coils  were  embedded,  owing 
to  the  generation  of  ozone,  it  is  supposed,  and  this  has  led  to  the 
breakdown  of  the  insulation.  Every  manufacturer  also  is  at  the 
mercy  of  a  careless  workman.  If  a  small  pin  point  is  left  in 
the  slot  of  an  armature,  it  will  work  its  way  through  the  insulation, 
and  sparking  will  take  place. 

There  is  never  any  difficulty  whatever  in  discovering  the  seat  of 
a  breakdown  of  insulation  in  a  continuous-current  armature.  It 
declares  itself  by  the  damage  it  does  at  the  point  where  the  insula- 
tion breaks  down.  When  the  insulation  breaks  down  between  the 
coils  and  the  core,  sparking  takes  place  between  them,  the  coils  being 
usually  welded  right  on  to  the  core,  and  there  is  nearly  always  a 


FAULTS   IN   ELECTRICAL   APPARATUS 


359 


considerable  liberation  of  heat  in  the  immediate  neighbourhood, 
leading  to  the  destruction  of  the  insulation  of  adjacent  coils.  Where 
the  insulation  between  two  adjacent  commutator  segments  breaks 
down,  there  is  also  no  difficulty  in  discovering  the  place.  The  mica 
plates  between  the  segments  are  usually  burned  away  in  very 
peculiar  forms,  sometimes  as  if  they  had  been  nibbled  by  rats.  The 
same  thing  applies  to  the  breakdown  of  the  insulation  between  the 
commutator  and  the  axle.  It  will  be  shown  usually  in  two  ways. 
The  coil,  or  possibly  more  than  one,  connected  to  the  segments  near 
which  the  breakdown  of  the  insulation  occurs  will  generally  be 
burned,  and  on  examining  the  insulating  ring  the  damage  will  be 
disclosed  immediately. 

Testing  for  Disconnection  in  Continuous- 
current  Armature 

Testing  for  a  disconnection  in  a  continuous -current  armature  is 
sometimes  necessary  when  the  break  refuses  to  declare  itself,  after 


WIRC 


BREAK*  MADE 
FDR  TEST 


Low  READINQ    • 
METER 


FIG.  159.—  Diagram  of  Connections  for  Testing  for  a  Break  in  the  Armature  Circuit. 


the  means  described  have  been  tried,  and  it  is  often  troublesome, 
because  there  are  two  circuits.  The  rule  is,  make  another  disconnection 


360  ELECTRICITY   IN    MINING 

by  unsoldering  a  pair  of  wires  at  one  of  the  commutator  arms,  seeing  that 
the  wires  are  brought  quite  clear  of  the  commutator  and  are  separated 
from  each  other,  then  form  a  circuit,  as  shown  in  Fig.  158,  with  a  dry 
cell  and  either  a  low-reading  ampere  meter  or  a  lineman's  galvano- 
meter, and  test  from  segment  to  segment  of  the  commutator  on  the 
side  of  the  disconnected  wire  to  which  connection  is  made.  The 
instrument  will  show  the  presence  of  a  current  as  each  segment  is 
touched,  providing  that  the  commutator  and  the  wire  from  the 
instrument  touching  it  are  clean,  and  firm  contact  is  made  between 
them,  till  the  disconnection  is  passed,  and  it  will  be  found  to  lie 
between  the  last  two  segments  touched.  Usually  a  further  examina- 
tion will  show  that  the  disconnection  is  at  the  junction  to  the 
commutator.  Occasionally,  however,  though  very  rarely,  the  dis- 
connection may  be  in  the  coil  itself,  between  the  two  segments  of  the 
commutator  last  touched.  Modern  dynamo  construction  is  so  well 
carried  out,  and  is  so  thoroughly  well  understood,  that  the  case  should 
very  rarely  arise  where  a  disconnection  in  any  part  of  the  coil  would 
be  found.  There  may  be  cases,  however,  in  old  dynamos,  or  where  a 
dynamo  has  been  repaired,  and,  in  place  of  putting  in  a  completely 
new  coil,  a  piece  has  been  soldered  on.  It  may  also  occasionally 
happen  that  a  bad  piece  of  wire  has  been  used,  in  spite  of  all  pre- 
cautions, and  it  has  parted  owing  to  the  vibration  of  the  machine. 
Usually  in  a  case  of  this  kind  sparking  also  takes  place  at  the  break, 
and  its  presence  will  be  shown  by  heat  of  that  coil.  In  any  case, 
however,  the  coil  indicated  must  be  replaced,  and  the  three  sets  of 
connections  to  the  commutator  and  any  equipotential  conductors 
properly  soldered  into  place. 


Turning  up  Commutators 

It  is  not  so  necessary  at  the  present  time  to  caution  users  of 
dynamos  about  turning  up  commutators  as  in  the  early  days  of  the 
dynamo.  The  commutator  requires  turning  up  very  much  less 
frequently  than  in  those  days,  and  it  is  now  better  understood  that 
care  must  be  taken  in  carrying  out  the  operation.  The  great  danger 
in  turning  up  the  commutator  is,  the  cutting  tool  that  is  employed 
always  carries  forward  a  portion  of  the  copper  it  is  turning  from  one 
segment  to  the  next,  in  the  direction  of  rotation.  Hence,  when  the 
turning  is  complete,  there  will  nearly  always  be  a  number  of  adjacent 
segments  connected  by  minute  pieces  of  copper  that  are  very  hard  to 
see,  except  by  the  practised  eye.  The  rule  is,  after  turning  up,  to 
carefully  clean  out  the  divisions  between  the  sections  with  a  specially 
sharp-pointed  tool,  and  to  go  over  and  over  several  times.  There  is, 
unfortunately,  no  test  that  can  be  employed  at  a  mine  that  will  show 


PLATE  30A.—  Heenan  &  Froude's  Fan,  for 
Mining  Work,  without  the  Case. 


PLATE  30B. — Heenan  &  Froude's  Fan,  in 
Case,  with  evase  Chimney,  driven  by 
an  Electric  Motor. 


PLATE  30c.—  Fan  House  at  one  of  the  Lambton  Collieries,  with  one  of  the  Fans 
driven  by  a  Three  Phase  Motor.  The  Wires  conveying  the  Current  are 
shown  entering  the  Tower  of  the  Fan-house.  By  Messrs.  Bruce,  Peebles  &  Co. 

[TofaceiJ.  360. 


FAULTS   IN   ELECTRICAL   APPARATUS  361 

when  two  commutator  sections  are  in  connection.  Perhaps  the  best 
test  is,  after  turning  is  complete,  and  after  the  divisions  have  been 
very  carefully  cleaned  out,  the  attendant  having  gone  over  them 
several  times,  to  put  the  armature  in  the  machine,  if  it  has  been 
taken  out,  and  to  run  it  slowly,  so  as  to  generate  a  comparatively  low 
pressure,  and  to  carefully  examine  the  coils  while  it  is  running,  and 
after  it  has  been  running  a  certain  time,  stopping  the  machine  for 
the  purpose.  If  a  connection  still  exists  between  two  adjacent  seg- 
ments, the  fact  is  generally  soon  known  by  the  coil  that  is  short- 
circuited  warming,  and  by  sparking  at  that  section.  A  plan  that  is 
designed  to  avoid  all  this  trouble  is  to  employ  one  of  the  machines 
that  are  on  the  market  for  turning  up  the  commutator  in  situ,  by 
means  of  either  emery  wheels  or  stones,  preferably  the  latter.  Care 
is  of  course  necessary,  with  this  method  as  with  the  other,  to  clear  off 
all  dust  that  is  formed  from  the  commutator,  the  radial  bars,  and 
other  parts  of  the  machine.  It  should  also  be  mentioned  that  dust, 
either  copper  or  carbon,  that  is  allowed  to  collect  between  the  com- 
mutator segments  or  on  the  insulation  of  the  brush  holders,  will 
sooner  or  later  lead  to  trouble. 

The  burning  out  of  a  coil,  which  occurs  when  two  adjacent  seg- 
ments of  a  commutator  are  connected,  is  due  to  the  fact  that  the  coil 
itself,  being  then  short-circuited  across  the  insulation  of  the  com- 
mutator, presents  a  very  low  resistance  indeed  to  the  pressure  that  is 
created  in  it  as  it  passes  through  the  magnetic  field.  The  pressure 
created  in  each  coil  is  not  large.  It  may  not  be  more  than  one  volt 
in  a  well-designed  machine ;  but  a  pressure  of  one  volt  when  opposed 
by  a  resistance  of  say  '001  ohm  furnishes  a  current  of  1000  amperes, 
and  the  heating  effect  being  as  the  square  of  the  current  strength, 
the  result  is  at  once  apparent. 


Testing  for  Disconnection  in  an  Alternating- 
current  Armature 

The  testing  of  an  alternating-current  armature  for  disconnection, 
whether  single,  two,  or  three  phase,  is  usually  a  very  simple  matter. 
With  single-phase  currents  a  dry  cell  and  a  detector  galvanometer, 
or  low  reading  ampere  meter  connected  to  the  collector  rings  on  the 
armature  shaft,  with  the  collector  brushes  thrown  back,  will  show  at 
once  if  there  is  a  disconnection.  If  a  circuit  is  formed  with  the 
dry  cell  and  indicating  instrument  and  one  of  the  collector  rings,  the 
wire  forming  the  other  end  of  the  circuit,  being  touched  on  the 
junctions  to  successive  coils  of  the  armature,  will  quickly  declare 
where  the  break  is.  The  instrument  will  show  a  current  at  the 
end  of  each  coil,  until  the  coil  is  passed  in  which  there  is  a 


362  ELECTRICITY   IN   MINING 

disconnection,  and  it  will  be  in  the  coil  between  the  last  two  tests. 
Disconnections  do  not  often  occur  in  alternating-current  armatures. 
With  two-phase  armatures,  the  same  process  is  carried  out,  testing 
the  coils  of  each  phase  separately. 

With  three-phase  armatures,  star  connected,  the  same  process  is 
carried  out,  but  by  forming  a  circuit  with  the  dry  cell  and  indicating 
instruments  as  before,  and  connecting  to  each  two  of  the  three  collector 
rings.  If  all  is  in  order  there  is  a  complete  circuit  between  each 
two  of  the  rings,  formed  by  two  of  the  coils  connected  at  the  neutral 
point.  If  there  is  no  circuit  between  one  of  the  rings  and  either 
of  the  other  two,  there  is  a  disconnection  in  that  set  of  coils,  and 
it  is  to  be  found  by  testing  from  that  ring,  as  described  with 
single  phase. 

With  three-phase  mesh-connected  armatures,  the  testing  must  be 
carried  out  in  the  same  way  for  disconnection  as  with  a  continuous- 
current  armature,  the  points  of  connection  between  the  coils,  of  which 
the  winding  is  formed,  being  used  for  testing,  in  the  same  way  as  the 
commutator  segment  with  a  continuous-current  armature,  and  one  of 
them  being  broken,  as  explained  in  connection  with  the  continuous- 
current  machine. 


Conductivity  and  Insulation  Tests  in  Dynamos 

There  are  a  number  of  apparatus  on  the  market  for  making 
accurate  conductivity  tests,  and  the  electrician  in  charge  will  be  wise 
to  make  himself  familiar  with  them,  but  for  all  practical  purposes 
any  conductivity  tests  that  are  required  may  be  taken  with  a  few  dry 
cells,  and  a  low-reading  ampere  meter.  It  has  been  mentioned  in  the 
foregoing  pages,  that  a  dry  cell  and  a  low-reading  ampere  meter,  or  a 
lineman's  detector  galvanometer,  will  answer  in  a  great  many  cases, 
and  it  is  quite  correct.  But  for  greater  accuracy,  and  for  better 
informing  himself  of  the  condition  of  the  circuits  of  the  dynamo 
machines  under  his  charge,  the  electrician  would  be  wise  to  use 
instruments  that  will  show  him  a  certain  definite  marked  deflection, 
and  it  may  be  necessary  to  use  a  number  of  dry  cells  for  the  purpose. 
The  conductive  resistance  of  large  field-magnet  coils  may  be  high. 
It  is  no  unusual  thing  to  have  a  field  coil  having  1000  ohms  resistance. 
Taking  the  pressure  available  from  a  single  dry  cell  as  probably  not 
exceeding  one  volt,  the  current  passing  through  the  circuit  formed 
with  one  dry  cell,  the  ampere  meter,  and  a  field  coil  will  be  only  one 
milliampere.  Any  convenient  form  of  milliampere  meter  may  be 
employed  for  the  work,  but  a  sufficient  number  of  dry  cells  should  be 
employed  to  give  a  deflection  of  30  degrees  or  so  on  a  circular  dial 
scale,  if  one  is  employed,  so  that  any  difference  in  the  conductive 


FAULTS  IN  ELECTRICAL  APPARATUS 


363 


resistance  of  any  magnitude  is  easily  apparent.  Fig.  160  shows  the 
connections  for  testing  for  a  break  in  the  circuit  of  the  field-magnet 
coils  with  a  dry  cell  and  low-reading  ampere  meter.  In  practical 
work  it  is  rarely  necessary  to  know  the  conductive  resistance  of  field 
coils  within  close  limits,  but  it  should  be  known  when  the  resistance 
is  very  largely  increased,  and  when  it  is  very  largely  decreased.  It 
will  be  largely  increased  if  the  wire  is  partly  eaten  away,  and  it  will 
be  largely  decreased  if  the  insulation  has  broken  down.  Both  of  these 

FIELD  MAQNET  GORES 
HEUD  CoiL.3 


HELD  MAQNCT 


BREAK 


WIRE 


Low  READING 

METER 


FIG.  160. — Diagram  of  Connections  for  testing  for  a  Break  in  the  Field 
Magnet  Circuit. 

are  shown  in  the  manner  suggested,  and  if  more  accurate  tests  are 
desired,  one  of  the  forms  of  Wheatstone's  bridge  had  better  be  em- 
ployed. The  author  would,  however,  advise  practical  engineers  not 
to  make  a  fetish  of  the  Wheatstone's  bridge.  It  is  a  very  beautiful 
and  very  useful  instrument,  especially  in  the  forms  in  which 
modern  instrument  makers  supply  it,  and  an  electrician  who  can 
use  one  can  find  out  almost  anything  he  pleases  with  it,  but  it  is 
an  apparatus  that  requires  a  certain  amount  of  skill  in  using. 
The  indicating  galvanometer  attached  to  it  is  always  delicate,  and 


364 


ELECTRICITY   IN   MINING 


its  own  measurements  are  very  liable  to  be  upset,  and  rendered 
totally  inaccurate  by  a  little  dirt,  or  a  little  carelessness,  in  putting 
a  plug  in  its  place.  A  loose  plug  will  upset  the  most  careful 
measurement. 


Testing  for  Insulation 

As  explained  in  a  previous  part  of  this  chapter,  insulation  tests 
must  be  made  always  with  a  pressure  at  least  equal  to  that  of  the 
service.  That  is  to  say,  on  a  500  volt  service,  insulation  tests  should 
be  made  with  from  500  to  600  volts,  and  on  a  200  or  220  volt  service 
with  300  volts  or  thereabouts.  The  author  does  not  believe  in  testing 
with  very  much  higher  voltages  than  are  intended  to  be  employed  on 


COMMUTATOR 
/ 


ARMATURE 


K3H 


TESTING  GENERATOR 

FIG.  161. — Diagram  of  Connections  for  testing  the  Insulation  [Resistance 
of  a  Continuous-current  Armature. 


the  service,  even  when  a  dynamo  is  first  made.  In  his  opinion  it 
tends  to  create  possible  sources  of  failure  later  on.  The  very  best 
arrangement  for  testing  insulation  is  one  of  the  numerous  apparatus 
on  the  market,  in  which  a  small  dynamo  machine,  arranged  to 
generate  current  at  from  100  to  600  volts,  according  to  the  require- 
ments of  the  service,  is  used  with  what  is  practically  a  galvanometer, 
graduated  in  megohms.  A  circuit  is  formed  with  the  generator,  the 
resistance  indicator,  and  the  apparatus  whose  insulation  resistance  is 
to  be  measured ;  and  the  dynamo,  which  is  provided  with  a  handle  for 


FAULTS   IN   ELECTRICAL   APPARATUS  365 

driving,  is  turned  rapidly,  the -resistance  being  read  off  on  the  dial. 
The  apparatus  mentioned  are  sometimes  made  with  generator  and 
resistance  indicator  in  one  box,  but  more  frequently  the  generator  is 
separate.  Fig.  161  shows  the  arrangement  of  the  test  for  the  insu- 
lation resistance  of  armature  to  shaft.  There  are  various  other 
instruments,  including  the  Wheatstone's  bridge,  which  may  be 
employed  for  testing  insulation,  but  they  are  nearly  always  very 
much  more  delicate  than  the  apparatus  described,  and  unless  they 
are  made  to  the  employed  with  the  pressures  named,  the  indica- 
tions of  insulation  resistance  which  they  give  may  be  inaccurate. 


Trouble  with  the  Brush  Holders 

As  explained  in  the  description  of  dynamos,  the  modern  dynamo 
carries  several  sets  of  brushes  held  on  spindles,  which  are  supported 
by  a  circular  rocker,  carried  on  the  face  of  the  machine  or  on  the 
bearing,  each  spindle  being  insulated  from  the  rocker  by  collars  of 
insulating  material.  This  is  one  of  the  weak  points  of  the  continuous- 
current  machine,  inasmuch  as  the  construction  leaves  very  little 
room  for  thickness  of  insulating  material.  In  the  modern  machines, 
however,  it  is  not  often  that  trouble  arises  from  the  breakdown  of  the 
insulating  collar,  but  it  is  wise  to  remember  the  fact  that  if  copper  or 
carbon  dust,  or  even  coal  dust,  are  allowed  to  collect  upon  the  surface 
of  the  insulating  collar,  as  they  will  do  if  the  latter  is  damp  with  oil 
or  water,  a  path  is  gradually  formed,  across  which  a  current  is  thrown 
at  some  favourable  instant  when  the  pressure  of  the  service  rises  for  a 
moment,  the  current  so  passing  burns  up  the  path  of  dust,  and,  in 
burning  it,  destroys  the  insulating  collar.  To  avoid  this,  always  keep 
every  part  of  the  brush  gear  scrupulously  clean  and  free  from  dust. 

Another  source  of  trouble  that  is  sometimes  met  with  in  con- 
nection with  brush  holders  is — and  it  applies  also  to  the  collectors  of 
alternating-current  machines — a  disconnection  is  formed  at  some 
portion  of  the  brush  circuit  by  a  film  of  oil  or  dirt,  or  the  two  com- 
bined. When  a  machine  is  running  continuously,  it  sometimes 
happens  that  oil  finds  its  way  to  the  brush  spindles,  and  the  other 
points  where  connection  is  made  between  the  coils  of  the  machine  and 
the  brushes.  So  long  as  the  machine  is  running,  nothing  may  happen, 
because  the  screws  and  other  parts  being  in  their  place,  and  properly 
tightened  up,  the  oil  and  dirt  do  not  get  in  between ;  but  when  the 
machine  is  stopped,  and  possibly  the  brush  holders  removed  for 
trimming,  the  olirt  and  oil  may  extend  to  the  point  where  the  con- 
nection to  the  brush  holders  will  be  made,  or  the  brush  holders  may 
be  fixed  at  a  slightly  different  spot,  with  the  result  that  a  resistance 
is  interposed,  sufficient  to  prevent  the  machine  from  generating  the 


366 


ELECTRICITY   IN   MINING 


current  required  to  build  up  in  the  first  instance.  The  remedy  for 
this  is,  keep  all  parts  of  the  brush  holders,  etc.,  scrupulously  clean, 
and,  whenever  they  are  dismounted,  see  that  all  parts  where  connec- 
tions are  made  are  clean  and  bright. 


Faults  in  Cables 

Cables  are  subject  to  the  same  faults  that  have  been  mentioned, 
increase  of  conductive  resistance,  and  decrease  of  insulation  resistance, 
and  it  is  more  particularly  in  cables  that  the  one  often  leads  to  the 
other.  A  cable  in  a  damp  place  may  have  its  insulation  resistance 
gradually  lowered  by  the  water  penetrating,  and  afterwards  the  water 
which  has  passed  through  the  insulating  envelope  may  eat  away 


CABUE 


,  CONDUCTOR 


RESISTANCE 
INDICATOR 


^  INSULATING 

vysxV       ENVELOPE 

ARMOUR  OR 
LEAD  COVER  IN  Q 


FIG.  162. — Diagram  of  Connections  for  testing  the  Insulation  Resistance 
of  an  Armoured  or  Lead-covered  Cable. 


the  conductor,  gradually  increasing  its  conductive  resistance  until 
severance  is  complete.  This  may  take  place  even  with  the  very 
largest  cables,  though  it  is  more  likely  to  with  smaller  ones.  The 
best  safeguard  against  faults  of  both  kinds  are  the  tests  required 
by  the  recently  issued  Home  Office  Eegulations.  The  insulation 
resistance  of  each  length  of  each  cable  should  be  tested  periodically, 
with  one  of  the  apparatus  mentioned  and  the  test  recorded,  and  if 
any  particular  cable  shows  signs  that  its  insulation  resistance  is 
falling,  it  should  be  carefulty  examined.  Figs.  162  and  163  show 


FAULTS   IN   ELECTRICAL   APPARATUS  367 

the  connections  for  testing  the  insulation  resistance  of  armoured  or 
lead-covered  and  plain-covered  cables  respectively.  It  is  not  so  easy 
to  test  the  conductive  resistance  of  cables,  because  it  entails  discon- 
nections which  are  troublesome,  and  the  cables  themselves  are  of  such 
low  resistance  that  a  conductive-resistance  test  is  very  difficult  to 
make  accurately.  Probably  the  best  guide  in  the  matter  of  the  con- 
ductive resistance,  and  even  of  the  insulation  resistance,  is  a  careful 
watch  upon  the  volt  meters  and  ampere  meters  at  the  main  switch- 
board and  sub-switchboards,  assisted  by  tests,  as  often  as  possible,  at 
distributing  points,  motors,  and  so  on.  If  the  current  passing  out 
to  any  particular  district  through  a  set  of  feeders  increases  without 
any  apparent  reason,  if  the  motors  taking  current  from  the  particular 
feeders  are  doing  the  same  work,  and  there  are  the  same  number  of 

CABLE 

'/////////////////\ 

^CONDUCTOR 


iNSULATIfNQ 


RESISTANCE 
INDICATOR 


KN 

TESTING  GENERATOR 

FIG.  163. — Diagram  of  Connections  for  testing  the  Insulation  Resistance  of 
a  Cable  without  Armour  or  Lead  Covering. 

lamps,  of  the  same  power,  where  lamps  are  in  use,  and  the  current 
steadily  increases,  it  is  a  pretty  sure  sign  that  the  insulation 
resistance  is  decreasing.  The  rule  in  this  case  is  to  break  the  cable 
at  the  different  points  where  the  switches  are,  and  test  each  section 
for  insulation  separately.  If  one  section  shows  a  largely  decreased 
insulation  resistance  it  should  be  carefully  overhauled,  and,  if 
practicable,  divided  into  sections,  by  breaking  at  points  where 
convenient  and  testing  each  section  separately.  If,  as  more  fre- 
quently happens,  the  insulation  resistance  is  found  to  be  steadily 
decreasing  through  the  whole  length  of  the  cables,  it  means  that 
the  whole  of  the  insulating  envelope  is  deteriorating,  and  it  should 
be  carefully  watched,  and  the  cables  replaced  before  the  matter  has 
gone_too  far.  Another  source,  and  a  very  frequent  one,  of  trouble 


368  ELECTRICITY  IN   MINING 

in  connection  with  cables  in  mines  is  damage  to  the  cables  from 
falls  of  roof.  The  damage  may  take  different  forms.  The  two  or 
three  cables  may  be  completely  severed,  the  severed  ends  remaining 
bare,  and  often  an  arc  forming  between  them,  until  the  fuse  or  the 
circuit  breaker  operates.  There  is  no  difficulty  in  finding  this  fault, 
and  it  only  requires  careful  rejointing.  The  cable,  however,  may 
not  be  severed,  but  where  there  are  two  or  three  cables  in  one  the 
conductors  may  be  forced,  more  or  less,  through  the  insulating 
envelope,  and  the  insulation  resistance  between  them  will  be  lowered. 
Where  single-armoured  cables  are  employed,  it  very  frequently 
indeed  happens  that  a  fall  forces  the  armour  through  the  insulating 
envelope  to  the  conductor.  In  both  of  these  cases  the  connection 
between  the  conductors,  or  between  the  conductor  and  the  armour, 
may  be  completed  at  once,  or  it  may  be  only  partial,  and  the 
insulation  resistance  will  not  only  be  lowered,  but  the  resistance  to 
sparking  also,  with  the  result  that  at  some  later  time,  when  some 
change  takes  place  in  the  circuit,  as  when  a  large  motor  or  several 
motors  are  switched  off,  and  there  is  a  large  rise  of  pressure,  a  spark 
will  pass  between  the  conductors,  or  between  the  conductor  and  the 
armour,  connecting  the  two  together.  Sometimes  an  arc  will  be 
formed  after  the  spark  has  passed,  great  heat  being  generated  in  the 
neighbourhood,  the  insulation  being  damaged,  and  possibly  pit  props 
or  coal  dust  set  fire  to.  When  this  latter  occurs  there  is  no  difficulty 
in  finding  the  fault,  but  it  is  wise  to  endeavour  to  prevent  the  fault 
occurring,  and  to  carefully  test  any  section  of  cables  that  has  been 
subject  to  falls,  for  insulation,  as  soon  after  the  fall  as  possible. 
Careful  testing,  as  explained,  will  often  save  a  great  deal  of  trouble 
at  a  later  date.  If  the  insulation  test  shows  that  the  insulation  has 
been  considerably  lowered,  that  section  of  cable  where  the  fall  has 
taken  place  should,  if  possible,  be  immediately  renewed,  or,  at  least, 
a  piece  where  the  damage  is  known  to  have  taken  place,  that  has 
laid  right  under  the  stones  from  the  roof,  should  be  cut  out  and 
replaced. 

Finding  a  Short  Circuit  between  Cables 

If  the  service  is  properly  protected  by  fuses  and  circuit  breakers, 
these  will  come  into  operation  when  the  short  circuit  occurs,  but  it 
is  sometimes  a  little  difficult  to  find  the  exact  position  of  the  con- 
nection between  the  cables  forming  the  short  circuit.  The  electrician 
in  charge  of  the  plant  will  usually  have  local  knowledge  of  his  cables 
that  will  assist  him  in  locating  the  trouble;  but  where  the  local 
knowledge  is  not  sufficient,  the  following  may  be  useful.  A  con- 
ductive resistance  test  may  indicate  approximately  the  position  of 
the  "  short."  Thus,  if  there  are  two  cables  of  a  certain  size  going 


PLATE  3lA. — Reavell's  Air  Compressor  with 
Electric  Motor.  Portable  Form,  arranged 
for  placing  anywhere  in  bye. 


PLATE  31s. — Reavell's  Electrically  Driven  Air 
Compressor,  with  Variable  Speed  Motor, 
Starting  Switch,  and  Speed  Regulating 
Resistance  complete. 


PLATE  31c. — Blackett's  Underground  Coal 
Conveyor  with  Three  Phase  Motor.  The 
Trough  shown  on  the  Right  is  the  Conveyor. 


PLATE  3lD.— Blackett's  Underground  Coal 
Conveyor,  for  use  at  the  Face  of  the  Coal, 
driven  by  a  Continuous  Current  Motor.  The 
Coal  is  seen  coming  out  on  the  Conveyor 
and  tipping  into  the  Tub  at  the  End  of 
the  Road. 


[To  face  p.  368. 


FAULTS   IN   ELECTRICAL   APPARATUS  369 

down  the  shaft  and  along  the  workings  to  the  neighbourhood  of  the 
face,  each  thousand  yards  has  a  certain  conductive  resistance,  and  a 
test  with  a  low-reading  ampere  meter  and  a  dry  cell  will  give  a 
rough  indication  of  the  length  of  cable  between  the  point  where  the 
test  is  being  made  and  the  "  short."  Further,  the  system  that  has 
been  so  strongly  recommended  in  these  pages,  and  that  is  insisted 
upon  so  wisely  by  the  Home  Office  Kegulations,  of  switches  at  the 
pit  bottom,  and  at  different  distributing  points,  will  be  found  of 
immense  service  in  testing  for  a  fault  of  this  kind.  Thus,  if  the 
switches  at  the  pit  bottom  are  open,  and  it  is  found  on  switching  the 
feeders  on  at  the  switchboard  that  the  circuit  breaker  comes  out,  or 
that  there  is  a  heavy  throw  of  current  on  the  ampere  meter,  it  is 
pretty  clear  that  the  fault  is  in  the  shaft.  It  is  wise  in  a  case  of  this 
kind  not  to  wait  for  the  circuit  breaker,  but  to  watch  the  ampere 
meter.  If,  on  the  other  hand,  there  is  no  throw  of  current  on  the 
ampere  meter,  and  the  circuit  breaker  does  not  move,  it  is  clear  that 
the  fault  is  beyond  the  pit  bottom,  and  this  may  be  repeated  until 
the  section  in  which  the  fault  occurs  is  located,  when  a  more  careful 
examination  should  be  made,  and  a  more  careful  test  for  conductive 
resistance. 

Testing  Cables  for  Disconnection 

This  is  often  a  very  troublesome  matter.  The  capacity  test  given 
on  p.  370  is  of  service,  but,  as  will  be  explained,  the  apparatus 
employed  is  somewhat  delicate.  Disconnections  do  not  often  occur 
with  large  cables;  but,  on  the  other  hand,  it  is  perfectly  possible 
for  them  to,  especially  at  joints.  Jointing  large  cables  is  somewhat 
difficult  in  mines,  and  if  moisture,  especially  some  of  the  pit  water,  is 
allowed  to  be  present  when  the  joint  is  covered  up,  it  will  assuredly 
eat  the  conductor  in  two,  and  then  it  is  difficult  to  find.  A  conductive 
resistance  test  in  this  case  is  of  not  much  value,  because  if  the  sever- 
ance of  the  conductor  is  complete,  no  circuit  can  be  formed,  and  that 
is  where  the  capacity  test  comes  in,  as  no  circuit  in  that  case  is 
wanted. 

A  voltmeter  test  will  do  a  great  deal  in  locating  the  section 
where  the  break  is.  If  there  is  a  voltmeter  at  the  pit  bottom,  and 
it  shows  the  same  pressure  as  at  the  main  switchboard,  when  no 
current  is  passing,  it  will  be  evident  that  the  fault  is  not  in  the  shaft. 
This  had  better  be  confirmed  by  taking  the  pressure  when  the  normal 
current  is  passing,  if  possible,  as  the  passage  of  the  current  may 
break  down  the  fault.  Passing  outwards  from  the  pit  bottom  with  a 
portable  voltmeter,  a  test  at  the  end  of  the  next  section  with  the 
current  on  and  off  will  show  whether  the  fault  is  in  that  section,  and 
this  may  be  continued  at  the  end  of  each  section.  When  the  pressure 

2B 


370  ELECTRICITY  IN   MINING 

is  lost  between  the  ends  of  two  sections,  the  fault  will  lie  in  that 
section,  and  if  a  careful  examination  does  not  disclose  it,  the  section 
should  be  replaced.  This  method  applies  equally  to  continuous 
current  and  to  three  phase,  bearing  in  mind  that  there  should  be  a 
pressure  between  each  of  the  three  cables  of  a  three-phase  service, 
and  that  if  there  is  no  pressure  at  the  end  of  any  section  between 
one  of  the  cables  and  either  of  the  others,  there  is  probably  a  dis- 
connection in  that  cable  in  that  section.  But  it  is  sometimes 
troublesome,  and  even  dangerous,  to  test  in  this  way  on  a  medium, 
and  more  so  on  a  high  pressure  service,  because  of  the  danger  of 
shocks  to  the  operators  when  making  connection  to  the  cables  with 
the  voltmeter.  This  can  be  provided  for  by  means  of  insulating 
handles  attached  to  the  end  of  the  voltmeter  wires;  but  the  safer 
method  is  as  follows : — Pass  a  current  of  low  pressure  through  the 
cables  either  by  running  the  generator  slowly,  or  from  a  battery,  or 
from  any  convenient  source,  with  an  instrument  in  circuit ;  it  need 
not  register  amperes,  it  need  only  show  the  presence  of  a  current, 
and  the  cable  must  be  broken  into  sections,  as  before,  the  ends  of 
each  section  being  connected  together  so  as  to  form  a  circuit.  If,  for 
instance,  the  switch  at  the  pit  bottom  is  opened,  and  the  terminals  of 
the  cables  at  the  pit  bottom  switchboard  are  connected,  the  instru- 
ment will  show  a  current  if  the  cables  in  the  shaft  are  all  right. 
Having  ascertained  that  the  cables  in  the  shaft  are  right,  the  switch 
at  the  pit  bottom  would  be  closed,  the  next  section  thrown  in,  tested 
in  the  same  way,  and  so  on.  The  fault  will  be  in  the  section  last 
tested  where  the  indicator  shows  no  current. 

Tests  by  Electrostatic  Capacity 

The  question  of  electrostatic  capacity  has  been  brought  into 
mining  work  from  the  difficulty  of  carrying  out  the  Home  Office 
Regulations  in  the  matter  of  leakage  indicators,  with  three-phase 
services,  owing  to  the  capacity  of  the  cables.  The  author  would 
hardly  advise  mining  engineers  to  trouble  very  much  with  capacity 
tests,  but,  on  the  other  hand,  if  their  electrician,  or  any  one  about  the 
mine,  acquires  a  considerable  skill  with  electrical  instruments,  the 
capacity  test  in  the  case  of  a  disconnection  or  a  short  circuit  may 
save  some  time.  The  rationale  of  the  test  is  this.  Each  length  of 
cable  with  its  insulating  envelope  has  a  certain  electrostatic 
capacity  at  a  given  pressure.  The  electrostatic  capacity  is  measured 
by  charging  the  cable  from  any  convenient  source  of  continuous 
current  for  a  certain  time,  and  then  discharging  through  a  special 
form  of  galvanometer  made  for  the  purpose,  and  noting  the  throw  of 
the  galvanometer  needle.  A  formula  which  need  not  be  given  here 
gives  the  capacity  of  the  cable  under  test,  with  the  given  pressure 


FAULTS   IN   ELECTRICAL   APPARATUS  371 

and  galvanometer.  If  capacity  tests  are  made  upon  all  the  cables, 
and  a  disconnection  or  a  short  circuit  occurs,  the  capacity  test  will 
show  it,  and  the  lowered  capacity  of  the  cable  under  test  will  show 
approximately  the  point  where  the  fault  occurs.  This  is  one  method ; 
there  are  others  more  complicated,  and  involving  more  delicate 
apparatus. 

Faults  in  Switch  Gear 

Faults  are  caused  in  switches,  circuit  breakers,  etc.,  by  the  wear 
of  the  moving  parts  by  sparking  between  the  contact  surfaces,  and  by 
the  deposit  of  dust  upon  the  insulating  surfaces  separating  parts  of 
the  switch,  between  which  a  pressure  exists.  Care  is  the  great  secret 
in  avoiding  these.  Switches  should  be  carefully  watched,  and  as 
they  wear  they  should  be  either  taken  up,  if  their  construction 
allows,  or  their  contacts  replaced.  If  the  wear  is  allowed  to  go  on, 
sparking  will  take  place,  often  followed  by  arcing,  with  the  result 
that  the  switch  itself  may  be  destroyed.  If  dust  is  allowed  to  collect 
as  described,  it  may  and  has  happened,  that  at  some  moment  when 
there  is  a  temporary  rise  of  pressure  in  the  service,  a  current  passes 
through  the  dust,  burning  it  up,  generally  leaving  an  arc  behind  it, 
and  destroying  the  insulating  material  and  the  contacts,  etc.,  in  the 
neighbourhood.  When  these  things  occur  there  is  no  difficulty  in 
finding  them,  they  are  only  too  evident.  Carefully  dusting,  and  watch- 
ing the  conditions  of  the  contact  services  of  switches,  etc.,  conductive 
resistance  tests  made  from  time  to  time,  as  opportunity  offers,  will 
be  found  to  be  of  assistance.  The  same  useful  set,  the  dry  cell  and 
low-reading  ampere  meter,  will  be  found  of  value  for  this  purpose ; 
but  it  will  only  be  of  value  if  tests  are  made  when  the  condition  of 
the  switch  is  good.  If  the  electrician  tests  the  contact  resistance  of 
a  switch  by  means  of  the  apparatus  described,  by  noting  the  current 
passed  through  it  from  his  dry  cell,  and  he  tests  it  later  on  and  finds 
that  with  a  cell  giving  the  same  pressure  and  with  the  same  instru- 
ments the  current  is  less,  it  will  be  a  sure  sign  that  the  contacts  are 
wearing.  There  may  be  no  harm  done  to  them.  There  may  still 
be  sufficient  surface  in  contact  to  carry  all  the  current,  but  if  the 
wear  is  allowed  to  continue,  sparking  will  result  sooner  or  later,  and 
possibly  heating  at  the  surface  will  take  place  sooner. 

As  mentioned  at  the  beginning  of  this  chapter,  in  using  a  dry  cell 
for  the  tests  that  have  been  mentioned,  always  test  the  cell  itself 
before  proceeding  to  make  a  test  with  it.  It  will  not  matter  very 
much  if  its  pressure  has  decreased,  so  long  as  the  electrician  knows 
it,  in  the  great  majority  of  cases.  But  if  he  does  not  know  it,  he 
may  often  obtain  misleading  indications. 


INDEX 


Absorption  of  electricity  by  insulating 
envelope  of  cables,  17 

Accumulators,  10,  193 

Advantage  of  high  pressures  for  distribu- 
tion, 235 

Ageing  of  iron,  how  provided  against, 
206 ;  in  transformers,  205 

Air-cooled  transformers,  206 

Alternate-current  arc  lamps :  difference 
in  light  given  from  continuous  current, 
54 ;  difference  in  pressure  required 
from  continuous  currents,  54  ;  differ- 
ence in  wasting  of  carbons,  54  ;  general 
description,  54 

Alternating  and  continuous  currents, 
difference  between,  14 

Alternating-current  arc  lamps :  precau- 
tions for  working,  61;  use  of  chok- 
ing coils  and  compensators  with, 
60;  use  of  compensator  with,  65; 
worked  from  single-phase  service,  60  ; 
worked  from  two-  and  three-phase 
service,  60 

Alternating-current  generator :  descrip- 
tion of,  185;  difference  from  con- 
tinuous-current generator,  185;  the 
disc  form,  description  of,  186 

Alternating  current :  maximum  pressure 
present,  15 

Alternating  currents :  calculations  for 
Ohm's  law,  15 ;  changes  in,  15 ;  the 
cycle,  15  ;  the  cycle  or  period  denned, 
15  ;  definition  and  explanation  of,  14  ; 
laws  governing,  15 ;  maximum  cur- 
rents, 15;  maximum  pressure,  15; 
mean  heating  values,  15  ;  the  period, 
15 ;  periodicity,  definition  of,  16 ;  ratio 
between  maximum  and  effective  cur- 
rent, 15 ;  ratio  between  maximum  and 
effective  pressure,  15;  square  root  of 
mean  square,  15 

Alternator  with  drum  armature,  descrip- 
tion of,  187 


Alternators :  the  number  of  cycles  of, 
191 ;  number  of  poles  in,  191 ;  perio- 
dicity of,  191 

Aluminium  conductors,  objections  to  the 
use  of,  212 

Aluminium :  unsuitable  for  fuses,  266 ; 
use  of,  for  conductors,  212 

Ampere,  the,  5 

Angold  double-carbon  open -type  arc 
lamp  for  rectified  current,  55 

Angold  enclosed  arc  lamp,  with  adjust- 
able resistance,  51 

Angold  open -type  double-carbon  arc 
lamp,  53 

Application  of  electricity  for  driving 
machines  in  mines,  272 

Arc  lamp:  arrangement  of  supports  in 
engine  houses,  65 ;  arrangement  of, 
supports  in  sidings  and  open  spaces, 
65 ;  arrangement  of  supports  on  pit 
heaps,  65;  burning  away  of  carbons, 
46 ;  general  description  and  working 
of,  45  ;  insulation  of,  from  support,  65 ; 
iron  posts  for,  description  of,  65 ; 
lattice- work  poles  for,  66  ;  L.  E.  F. 
support  for,  67  ;  Schaeffer  contact  sup- 
port for,  67 ;  Schaeffer  safety  hook  for, 
description  of,  67 ;  use  of  flexible 
stranded  rope  for  hoisting,  65  ;  wooden 
posts  for,  65 

Arc  lamps :  continuous  current,  use  of 
resistance  with,  reason  for,  62 ;  fixing 
of  carbons,  46;  flickering  of  carbons, 
46 ;  forms  of,  in  use  in  mines,  45  ; 
working  of,  from  motor  generator, 
59 

Arc  lighting  by  continuous  -  current 
machines,  178 

Armature  coils,  connection  of,  to  com- 
mutator, 175 

Armouring  bitumen  cables,  218 

Armouring  cables:  objections  to,  230; 
the  question  discussed,  230 

Arrangement  of  apparatus  in  generating 
station,  206 


373 


374 


INDEX 


Arrangement  of  arc  lamps  :  to  work  100  to 
500  volt  services,  59;  on  65  to  110 
volt  services,  59 

Arrangement  of  cables  with  two-  and 
three-phase  currents,  242 

Arrangement  of  lamps :  in  parallel  series, 
8 ;  in  series  parallel,  8 

Asbestos-covered  fuses,  description  of,  267 

Atkinson's  time-limit  circuit  breaker, 
description  of,  270 

Atom,  the,  1 

Atomic  theory,  1 

Atomic  weight,  the,  1 

Attraction  between  positive  and  negative 
corpuscles,  2 

Automatic  cutouts  for  continuous-cur- 
rent arc  lamps,  description  of,  63  ;  use 
of,  with  continuous-current  arc  lamps, 
62 

Auto  transformers  for  three-phase  mo- 
tors, 279 


B 


Babcock  and  Wilcox  boiler :  description 
of,  91;  description  of  superheater, 
92 

Balancers,  description  of,  200 

Bar  coal-cutting  machines,  description 
and  operation  of,  326 

Batteries  employed  for  mining  signals, 
27 

Belliss  engine,  120 

Bells :  for  mining  signals,  35 ;  for  use  in 
damp  situations,  36;  for  use  in  ex- 
plosive atmospheres,  35 ;  for  use  in 
metalliferous  mines,  36 

Binding  wires  for  engine-road  signal 
wires,  30 

Bipolar  continuous-current  generators, 
description  of,  171 

Bitumen :  composition  of,  and  prepara- 
tion for  use  as  an  insulator,  218; 
covered  cables,  218 

Blackett's  coal  conveyer  for  the  coal 
face,  348 

Blast  -  furnace  gas :  cleaning  of,  153  ; 
rationale  of,  153  ;  the  use  of,  152 

Boiler,  circulation  of  feed  water  in,  im- 
portance of,  109 

Boiler  efficiency,  lowered  by  deposit  of 
salts,  108 

Boiler  feed,  by  injectors,  explanation  of, 
112 

Boiler-feed  pump:  electrically  driven, 
description  of,  111 ;  three-throw  type, 
111 ;  with  variable  stroke,  111 ;  Worth- 
ington  Co.'s,  109 


Boiler-feed  water:  containing  salts, effect 
of,  on  boiler,  108  ;  methods  of  supply- 
ing, 109 

Boiler  furnace,  temperature  of,  101 

Booster :  description  of,  197;  panel,  246  ; 
use  of,  196;  use  of,  for  charging  ac- 
cumulators, 196 

Bord  and  pillar  system  of  coal  working, 
325 ;  American  modification  of,  325 

Branch  circuits,  current  passing  in,  6 

Breaking  circuit,  6 

Bridge  fuses,  267 

Bringing  machines  into  service  and  taking 
out  with  bus  bars,  249 

B.  Th.  Unit :  definition  of,  99  ;  mechani- 
cal equivalent  of,  100 

B.  T.  H.  Co.'s  enclosed  arc  lamp,  51 

B.  T.  H.  Co.'s  high  tension  oil  immersed 
switch,  253 

Brockie-Pell  brake,  mechanism  for  arc 
lamp,  working  of,  48 

Brushes,  the  office  of,  in  commutating, 
183 

Brush  gear  for  bipolar  machines,  181 ; 
multipolar  machines,  181 

Brush-holders :  collection  of  dust  on  in- 
sulation, and  faults  resulting  from, 
365 ;  conditions  they  must  conform  to, 
184 

Bucket  pump,  description  of,  296 

Bulkhead  fitting  for  incandescent  lamps 
for  use  underground,  75 

Burning  out  of  armature  coils,  rationale 
of,  361 


Cables  for  coal-cutting  machines  and 
moving  motors,  construction  of,  229 

Callender's  bitumen  cables,  construction 
of,  218 

Calorific  values  of  different  coals,  101 

Campbell  Gas  Co.'s  oil  engine,  description 
of,  164 

Campbell  oil-engine  governor,  165 

Campbell  suction  gas  producer,  156 

Capacity  of  cables,  17 

Capel  fan,  description  of,  337 

Carbon  brushes :  density  of  current  with, 
183  ;  for  continuous-current  machines, 
182 

Carbon  filament :  behaviour  of,  when 
current  passes,  72 ;  disintegrating 
action  in,  with  current,  72 

Cathode,  2 

Causes  of  failure,  general  notes  on,  350 

Centiampere :  meter  for  general  road- 
signals,  32 ;  use  of,  in  testing,  354 

Central  condensing  stations,  141 


INDEX 


375 


Centrifugal  fan,  description  of,  337 

Centrifugal  governor,  description  of,  126 

Centrifugal  pump :  arranged  in  series, 
291 ;  arranged  for  sinking  with  electric 
motor,  292 ;  best  output  with  con- 
stant speed,  291 ;  best  speed  for 
efficiency,  290;  conversion  of  velo- 
city head  into  pressure  head,  288 ; 
description  of,  287  ;  efficiency  of,  287  ; 
forms  now  made,  288 ;  modern  con- 
struction to  avoid  eddies,  288 

Chain  coal-cutting  machines,  description 
and  operation  of,  326 

Charge  made  by  cables  for  converting 
energy,  209  ;  on  pressure,  210 

Chimney,  replaced  by  fan,  105 ;  the  use 
of,  104 

Choking  coils  and  compensators  for 
alternating-current  arc  lamps,  63 

Chopper  switches,  description  of,  265 

Circuit  breakers :  construction  of,  268  ; 
use  of,  268 

Classification  of  engine  by  expansion,  120 

Climax  water-tube  boiler,  description  of. 
96 

Closed  circuit,  6 

Coal  conveyers :  the  bucket,  the  belt,  and 
the  archimedian  screw,  118 ;  general 
arrangement  of,  118 

Coal  cutting  by  electricity,  324 ;  with 
drilling  machines,  working  of,  335 

Coal-dust  burning  engine,  description  of, 
168 

Coke-oven  gas :  calorific  value  of,  153 ; 
production  of,  153  ;  quantity  required 
per  H.P.  per  hour,  155 

Combustion :  explanation  of,  98  ;  of  coal, 
heat  liberated  by,  98;  of  petroleum, 
heat  liberated  by,  98 

Commutator  insulating  rings,  form  and 
construction  of,  175 

Comparison  of  volt :  with  pressure  of  Le- 
clanehe  cell,  5 ;  with  pressure  of 
Standard  Clark  cell,  5 

Compensator  enables  alternating-current 
arcs  to  be  extinguished  without  loss, 
65 

Compound  engine,  description  of,  121 

Compound  and  triple-expansion  engines, 
work  done  by,  122 

Compound,  triple,  and  quadruple  engines, 
division  of  work  in,  124 

Compound  -  wound  continuous  -  current 
generator :  description  of,  180  ;  com- 
pounding up,  explanation  of,  181 

Compound- wound  motor,  forms  of,  275 

Compressed  air:  inefficient  use  of,  in 
motor  engine,  343 ;  loss  by  heat  gene- 
rated, 342 ;  losses  in  transmission  of, 
341 ;  moisture  present  in,  343 ;  plant, 


description  of,  342 ;  plant  driven  by 
electric  motor,  description  of,  344 

Concentric  cables :  description  of,  221  ; 
with  uninsulated  outers,  223 

Condensers  :  advantages  of,  134  ;  cooling 
water  required  with,  138;  ejector, 
Ledward,  138 ;  evaporative,  Ledward, 
136;  forms  of,  135;  general  descrip- 
tion of,  135 ;  jet  and  ejector,  disad- 
vantages of,  139 ;  jet,  Worthington, 
137;  surface,  difficulties  with,  139; 
when  not  economical,  135 

Conditions  for  small  sparking  at  com- 
mutator, 184 

Conductivity  and  insulating  tests  in 
dynamos,  362 

Conductors  for  electric  light  and  power 
distribution,  211 

Conductors  and  insulators,  5  ;  use  of  old 
wire  ropes  for,  211 

Connecting  engine-road  signal-wires  at 
junctions,  31 

Connecting  separately  excited  machines 
to  bus  bars,  247 

Connecting  shunt-wound  machines  to 
bus  bars,  247 

Connections  of  coils  in  ring  and  early 
drum  armatures,  173 

Conservation  of  energy,  3 

Contact  supports  for  arc  lamps :  reason 
for  use  of,  66 ;  general  description,  66 

Continuous-current  armatures,  insula- 
tion of  conductors,  171 

Continuous-current  commutator,  con- 
struction of,  174;  difficulties  in  insu- 
lating, 174 

Continuous  current,  definition  of,  14 

Continuous-current  generators,  169  ;  the 
armature,  construction  of,  170 

Continuous-current  machines :  with  corn- 
mutating  poles,  description  of,  185  ; 
with  two  armature  windings,  200 

Continuous-current  motor,  description 
of,  272 

Converter  equalizer  electric  winding  ap- 
paratus, 317 

Conveyers  for  ashes,  119 

Cooling  apparatus,  difficulties  met  with 
in  working  of,  151 

Cooling  stationary  transformers,  205 

Cooling  towers,  operation  of,  147 

Cooling  towers  and  ponds,  reasons  for 
employment  of,  146 

Cooling  transformers,  use  of  cold  storage 
plant,  206 

Copper  brushes :  density  of  current  with, 
183  ;  use  of,  183 

Copper  losses  in  stationary  transformers, 
204 

Core  transformer,  description  of,  202 


37^ 


INDEX 


Cornish  boiler,  89 

Corpuscular  theory  :  of  electric  current, 

2  ;  of  light,  2 
Crook's  tubes,  2 
Current  for  mining  signals  from  power 

and  lighting  service,  27 
Curtis  turbine,  description  of,  144 
Cyclone  method  of  burning  coal  dust, 

IpT 
Cylinder    condensation    discussed,   127 ; 

in    compound    and    triple    expansion 

engines,  127 ;  methods  of  overcoming, 

128 


Dashpots,  employment  of,  in  arc  lamps, 
53 

Definition  of:  the  ampere,  6;  B.  Th. 
Unit,  9 ;  lines  of  force,  12 ;  magnetic 
reluctance,  13 ;  magnetic  resistance, 
13 ; -the  Ohm,  5  ;  specific  heat,  9 ;  two- 
phase  currents,  16;  the  volt,  5;  the 
watt,  9 

De  Laval  turbine,  description  of,  143 

Delivering  alternating  current  to  arc 
lamps,  general  arrangement  for  100  to 
500  volt  services,  60 

Delivering  current  to  arc  lamps,  59  ;  to 
coal-cutting  machines,  329 ;  to  incan- 
descent and  Nernst  lamps,  general  in- 
structions, 79 

Denver  Engineering  Go's,  electric  air 
drill,  332 

Dialite,  advantages  claimed  for,  218 

"  Dialite  "  bitumen  cables,  construction 
of,  218 

"Diatrine"  insulation,  description  of, 
219 

Diesel  engine  :  control  of  the  fuel  supply, 
168  ;  description  and  operation  of,  166 ; 
government  of,  168 

Difference  in  the  working  of  continuous 
and  alternating  current  arc  lamps,  61 

Different  forms  of  lamp-holders,  descrip- 
tions of,  73 

Difficulties  of  working  expansively,  126 

Direction  of  lines  of  force,  12;  in  the 
dynamo  machine,  12 

Disc  alternator:  connection  of  coils  in, 
186;  the  exciter  for,  arrangement  of, 
187 ;  disc  coal-cutting  machines, 
description  and  operation  of,  326 

Disconnections  in  continuous  -  current 
armatures,  358 

Distribution  by   two-  and    three-phase 
currents,  241 ;  at  high  tension,  243 ;  at 
extra  high  tension,  244 
Distribution   of    light  with    alternating 


and  continuous  current  arc  lamps,  54 ; 
of  power  by  electricity,  209 

Double-carbon  arc  lamps,  general  descrip- 
tion and  working  of,  52 

Double-pole  switches,  264 

Draught :  by  steam  jet,  operation  of,  105  ; 
explanation  of,  104 

Drilling  by  electricity,  330 

Driving  air  compressors,  341 

Drum-armature  alternator,  connections 
of,  188 

Dry  cell,  description  of,  29 

Dynamo  machine,  general  arrangement 
of,  11 

Dynamotor,  description  of,  198 


E 


Early  drum  armature,  description  of, 
172 

Early  Siemens  armature,  172 

Economizers,  113 ;  Carter's,  114 ;  cleaning 
the  outside  of  tubes  of,  114 ;  deposit  of 
soot  on  the  outside  of,  114  ;  driving  the 
scrapers  for,  114 ;  Green,  114 

Edison  secondary  battery,  description  of, 
194 

Edwards'  air  pump  for  condensers,  de- 
scription of,  135 

Effective  amperes,  15 

Effective  volts,  15 

Effect  of  heat  upon  resistance  of  cables, 
227 

Efficiency  and  life  of  incandescent 
lamps,  71 

Electric  air  percussion  drills,  332 

Electric  circuit  :  definition  of,  6 ; 
measurement  of  the  power  in,  209 

Electric  condenser  return  current,  time 
interval  with  main  current,  18 

Electric  drilling  machine,  forms  of,  332 

Electric  driving  of  :  bucket  pumps,  297  ; 
other  machinery  about  a  mine,  347; 
overhead  rope  railways,  309 

Electric  fuses  for  shot  firing,  41 ;  connect- 
ing in  parallel,  43  ;  connecting  in  series, 
43 ;  high  tension,  description  of,  41 ;  high 
tension,  pressure  required  for,  42 ;  low 
tension,  battery  for,  41;  low  tension, 
current  taken  by,  41;  low  tension, 
description  of,  41;  magneto-exploder 
for,  description  of,  42 

Electric  ignition  for  gas  and  oil  engines : 
with  battery,  166;  with  magneto- 
generator,  166 

Electric  locomotives,  346 

Electric  locomotive :  for  use  on  the  sur- 
face, 347;  on  overhead  rope  railway, 
309 ;  trolleys  employed  with,  347 


INDEX 


377 


Electric  mining  signals,  forms  used,  22 

Electric  motor,  forms  of,  272 

Electric  motors :  advantages  of,  for  iso- 
lated positions  over  steam,  348 ;  must 
not  be  started  too  quickly,  277 ;  and 
their  switchboards,  348 

Electric  percussion  drills,  332 

Electric  rotary  drilling  machines,  332 

Electric  storage  battery  locomotive,  346 

Electrical  condenser,  18 

Electrical  current,  4 

Electrical  pressure,  4 ;  by  contact,  10 ; 
from  electrostatic  return  current,  18 

Electrical  pressures  set  up  between 
copper  and  lead,  10;  between  iron 
and  copper,  10. 

Electrically  driven  coal  conveyers,  348 

Electrically  driven  dip  pumps,  287 

Electrically  driven  fans  v.  steam-driven, 
340 ;  place  for,  340 

Electrically  driven  pumps,  286  ;  at  Tra- 
falgar Colliery,  286 

Electrically  driven  sinking  pumps,  287 

Electrically  driven  and  steam  driven 
compressed  air  compared,  344 

Electrolysis :  the  anode,  20 ;  the  cathode, 
21 ;  effect  upon  rails,  ropes,  pulleys, 
etc.,  21 ;  in  the  Leclanche  battery, 
21 ;  in  the  secondary  battery,  21 ;  with 
leakage  currents,  21 

Electro-magnetic  induction,  19 ;  with 
alternating  current,  19  ;  with  coils,  19  ; 
with  continuous  current,  19 ;  with 
straight  cable,  19 

Electrons,  2 

Electrostatic  capacity :  effect  of  decrease 
of  pressure,  18 ;  effect  of  increase  of 
pressure,  18 ;  of  long  cables,  18 

Electrostatic  capacity  and  leakage  cur- 
rents in  three-phase  services,  17 

Electrostatic  charge,  explanation  of,  17 

Electrostatic  induction,  explanation  of, 
17 

Electrostatic  return  current  to  the  cable, 
18 

Electrostatic  voltmeter 
262 

Electrotonic  theory,  1 

Enclosed  arc,  45 

Enclosed  arc  lamp :  forms  in  which 
carbon  burns,  46  ;  general  description 
and  working  of,  51 

Enclosed  fuses,  description  of,  267 

Enclosed  iron  switches  for  underground, 
258 

Endless  rope  haulage,  description  of,  300 

Energy  of  coal,  methods  of  employing, 
88 

Engine-road  indicator  signal,  description 
of,  24 


Engine-road  signal,  description  of :  with 
bell  at  each  end  of  road,  23 ;  with  one 
bell,  22  ;  with  two  bells,  23 

Engine-road  signals,  and  cables  for  light- 
ing and  power  service,  important  note 
respecting,  33 

Engine-road  signals,  wire  employed,  30 

Engine-road  signal  wires:  not  to  touch 
cables,  33  ;  termination  of,  31 

Engine-road  signal  with  bells  at  several 
stations,  connected  in  parallel,  24; 
connected  in  series,  24 

Equalizing  bar,  use  of,  in  paralleling, 
251 

Estimating  power  of  steam  and  com- 
pressed air  engines,  348 

Ether,  2 

"  Excello  "  flame  arc  lamp,  55 ;  carbons 
employed  in,  55 ;  comparison  of  light 
given  with  ordinary  arc  lamp,  57 

Excitation  of  continuous-current  ma- 
chines, 175 

Expansion  governor,  description  of,  126 


Failure  by  lowering  of  insulation  resist- 
ance, 350 
Failure  by  resistance  in  conductive  path, 

350 

Fall  of  pressure  in  cables,  210,  224 
Faults :  in  armoured  cables  from  falls  of 

mineral,  368;  in  cables,  366 ;  in  dynamo 

machines,  general  notes  on,  357 ;  in 

mine  signals,  352 ;  in  mining  bells,  354  ; 

in  switch  gear,  371 ;  in  telephones,  355 
Feed-water  heaters :  general  description 

of,  114  ;  Eoyle's,  115 
Ferranti  alternating  current  time-limit 

relay  for  circuit  breakers,  description 

of,  269 
Finding  a  short  circuit  between  cables, 

368 
Fireproof  covering  for  cables :  description 

of,  220 ;  reason  for  use  of,  220 
Firing  shots  by  electricity,  41 ;  operation 

of,  42 ;  precautions  with,  42 
Fitting  up  engine-road  signals,  30 
Fittings    for     high    C.P.    incandescent 

lamps,  76 

Five-wire  system,  explanation  of,  239 
Fixing  cables :  in  mines,  229 ;  in  shafts, 

230;  on  engine  roads  in  wood  boxes, 

234 
Fixing  wires  on  engine  roads,  methods 

of,  234 
Flame  arc  lamps :  the  carbons  employed, 


INDEX 


54  ;  colour  of  the  light,  how  produced, 
54 ;  general  description,  54 ;  spreading 
of  flame  by  electro-magnet,  55 

Flues  of  steam  boilers,  use  of,  89 

Forced  draught :  by  steam  jet,  105 ; 
explanation  of,  105 

Formation  of  smoke,  explanation  of,  102 

Formation  of  stranded  conductors, 
explanation  of,  221 

Forms  in  which  Ohm's  law  may  be 
written,  4 

Forms:  of  fans,  description  of,  336;  of 
galvanic  battery,  28;  of  pumps,  287; 
of  starting  gear  for  motors,  276 

Formula:  for  current  passing  in  main 
circuit,  6 ;  for  heat  liberated  by  electric 
currents,  9 ;  for  heat  liberated  in  cables, 
226 ;  for  measurement  of  work  in  alter- 
nating currents,  9 ;  for  rate  of  doing 
work,  9 

Four-phase  currents,  17 

Friction  of  water  in  pipes,  equivalent 
head  for,  298 

Frictional  electric  firing  apparatus  :  diffi- 
culties in  the  use  of,  44;  general 
description  of,  43 

Furnaces  of  steam  boilers,  description 
of,  89 

Fuses :  construction  of,  266 ;  enclosed  in 
iron  cases  for  mining  work,  268  ;  the 
use  of,  266 


G 


Gas  detector  for  portable  electric  lamps : 
H.  J.  Prested's  apparatus,  82 ;  Suss- 
mann  form  of,  82 

Gas  engine  working  with  suction  pro- 
ducer gas,  description  of,  155 

Gas  engines  :  combustion  of  gas  in,  158  ; 
for  large  powers,  161  ;  ignition  of 
explosive  mixture,  158 

Gas  and  oil  engines,  arrangement  of 
water  jacket  and  water  reservoirs,  160 ; 
cooling  the  cylinders  of,  160;  the 
government  of,  159 

Gate  road  connecting  boxes,  construction 
of,  259 

Gem  carbon  filament  incandescent  lamp, 
description  of,  70 

Generating  station,  size  of  units,  207 ; 
units  in,  206 

Generation  of  electricity  by  water  power, 
83 ;  forms  of  power  that  can  be  em- 
ployed, 83 

Generators  of  electricity :  forms  of,  169 ; 
principles  upon  which  all  are  con- 
structed, 169 


Government  of  compound  and  triple- 
expansion  engines,  124  ;  steam  engines, 
methods  of,  126 

Gramme  armature,  172 

Gravity  machinery  instruments,  descrip- 
tion of,  261 

Grease  extractors,  necessity  for  and 
general  description  of,  117 

Guards  for  overhead  conductors,  215 

Gutermuth  valve  for  ram  pumps,  descrip- 
tion of,  293 

Gutta  percha  for  insulating  cables,  objec- 
tions to,  216 


Haulage  arrangement  for  Longwall  coal- 
cutting  machines,  327 

Heading  machines :  description  of,  327  ; 
operation  of,  328 

Heat :  delivery  of,  to  water  in  a  steam 
boiler,  99 ;  is  one  form  of  work,  9 ; 
liberated  by  combination  of  carbon 
and  oxygen,  101 ;  liberated  by  electric 
currents,  9 ;  loss  of,  due  to  hot  gases 
in  motive  column,  105 ;  loss  of,  from 
presence  of  nitrogen  in  air,  101 ;  loss 
of,  when  furnace  doors  are  open,  103 ; 
unit,  9 

Heating  effect  on  short  circuit,  227 

Heating  of  cables,  226 

Heavy  cables,  makers'  lengths  of,  221 

High  C.P.  incandescent  lamps :  forms 
made,  efficiencies  of,  71 ;  general 
description,  71 

High  efficiency  lamps,  when  it  is  cheaper 
to  burn,  71 

High  pressure  insulators,  resistance  to 
sparking  of,  213 

High  tension  cell  switches,  description 
of,  253 

High  tension  insulators,  supporting  bolts 
of,  214 

High  tension  oil  immersed  switches, 
description  of,  253 

Hit-and-miss  governor,  159 

Holders  and  fittings  for  incandescent 
lamps,  72 

Holders  for  high  C.P.  incandescent 
lamps,  76 

Holing  or  kirving,  description  of,  324 

Hornsby  oil  engine  :  description  of,  163  ; 
governor,  165 

Hornsby  water-tube  boiler,  description 
of,  94 

Hot  wire  measuring  instruments,  descrip- 
tion of,  261 


INDEX 


379 


Ignition  by  electric  spark  in  gas  and  oil 
engines,  description  of,  166 

Ignition  of  explosive  atmospheres  from 
trembler  bells,  35 

Impressed  pressure;  how  calculated,  20; 
in  inductive  circuits,  20 

Incandescent  lamps :  B.C.  lamp,  68 ; 
carbon  filament,  the  light  given  by, 
69 ;  carbon  filaments,  pressures  made 
for,  69 ;  carbon  filaments,  sizes 
made,  69;  C.C.  lamp,  68;  connecting 
in  series,  objections  to,  80 ;  current 
taken  by,  at  different  pressures  and 
efficiencies,  69;  economy  of  burning 
under  power,  72 ;  efficiency  of,  69 ; 
forms  employed  in  mines,  68 ;  low 
pressure  and  high  pressure  compared, 
69 ;  vitrite  cap,  68 ;  winking  in  with 
alternating  currents,  70 ;  with  carbon 
filament,  high  C.P.,  70;  with  carbon 
filament,  low  C.P.,  70 ;  with  metallic 
filament,  70 

Incandescent  metallic  filament  lamps, 
70. 

Increase  of  temperature  in  a  cable,  10 

Independent  distribution  system :  advan- 
tages of,  252 ;  disadvantages  of,  252 ; 
working  of,  251 

Indiarubber  for  insulating  conductors, 
216;  objections  to  the  use  of,  217; 
Para  v.  W.  African,  217 

Induced  draught :  difficulties  in  arrange- 
ment of,  105 ;  explanation  of,  105 

Inductive  pressure  at  right  angles  to 
service  pressure,  19 

Inductor  alternator,  description  of,  192 

Infra  red  rays,  3 

Injector :  boiler  pressure  it  will  feed 
against,  113;  Holden  &  Brooke's, 
113 ;  use  of,  with  exhaust  steam,  113 

Insulated  conductors,  215;  substances 
employed  for,  215 

Insulating  substances  for  cables,  com- 
position of,  216 

Insulators :  for  engine-road  signals,  30 ; 
for  overhead  conductors,  213 ;  sub- 
stances forming,  5 

Intercommunication  telephone  appa- 
ratus :  description  of,  38 ;  operation  of, 
39  ;  wires  for,  39 

Internal  combustion  engine,  general 
description  of,  157 

Iron  boxes  for  enclosing  switches  for 
underground,  258 

Iron  losses  in  stationary  transformers, 
204 

Iron  nickel  battery,  193 


Iron  poles  for  overhead  conductors,  215 
Isolated    engines,  displacement    of,    by 
electric  motors,  348 


Jandus  arc  lamp,  51 

Johnson  &  Phillips'  enclosed  arc  lamp, 

description  of,  52 
Jointing  shaft  cables,  234 
"  Juno  "  flame  arc  lamp,  description  of, 

56 


Kinds  of  lamps  in  use  in  mines,  45 
Knife-blade    switches,  construction    of, 

263 

Korting  cooling  pond  with  nozzle,  151 
Korting  gas  engine  :  description  of,  161 ; 

quantity  of  gas  consumed  by,  162 


Lag  of  current  behind  pressure,  19 

Lahmeyer  Co.'s  arrangement  of  three- 
phase  motor  for  main  and  tail  haulage, 
306 

Lamp-holder  for  small  incandescent 
lamps,  description  of,  72 

Lancashire  steam  boiler,  88;  Messrs. 
Galloway's  modification,  89 

Large  gas  engines  with  two  Otto  cylinders 
in  tandem,  161 

Latent  heat :  explanation  of,  100 ;  of 
steam,  100 ;  of  water,  100 

Law  of  electrostatic  capacity,  17 

Lead  lead-oxide  battery,  description  and 
operation  of,  193 

Leakage  circuits,  6 

Leakage  current  shown  by  centiampere 
meter,  32 

Leclanch6  battery,  open  type,  descrip- 
tion of,  28 

Leclanch6  cells,  hints  on  recuperating, 
352 

Length  of  waves:  in  red  rays,  2;  in 
violet  rays,  2 

Lighting  service  fron  three-phase  power 
service,  243 

Lineman's  galvanometer,  description  of, 
352 

Liquid  resistance :  for  starting  gear,  276 ; 
for  varying  speed  of  series  motor,  dis- 
advantages of,  283 

"  Locked  coil  "  armour  for  cables,  219 


380 


INDEX 


Longwall  coal-cutting  machines,  de- 
scription of,  325;  operation  of,  326; 
speed  of,  327 

Longwall  system  of  coal  working,  325 
Longwall  system  of  coal  cutting,  324 
Lord  Kelvin's  law :  of  loss  in  cables,  225 ; 
applied  to  cheap  sources  of  current, 
225 

Loss  in  cables  generally  allowed,  225 
Low  efficiency  lamps,  when  it  is  cheaper 

to  burn,  72 

Lower  carbon-holder  of  arc  lamps,  ar- 
rangement of,  52 
Luna  arc  lamp,  description  of,  49 
Luna  shunt  arc  lamp,  working  of,  49 


M 


Magnets,  attractions  and  repulsions  of 

poles  of,  12 

Magnetic  circuit:    of  the  dynamo  ma- 
chine, 13  ;  of  electric  bells,  14 ;  law  of 

the  resistance  of,  14 
Magnetic    flux,    density,    and     ampere 

turns,  13 

Magnetic  lines  of  force,  12 
Magnetic  permeability  of  bismuth,  13 
Magnetic  permeability  of  Swedish  iron 

and  special  magnetic  steel,  13 
Magnetic  reluctance,  13 
Magnetic  reluctance  of  air  and  insulating 

material,  13 
Magnetic  reluctance  of  iron  and  steel, 

13 

Magnetic  resistance,  13 
Magnetism  and  ampere  turns,  13 
Magnetism:  arrangement  of  wire  coils 

for  creating,  by  electric  currents,  13 ; 

excitation  of,  in  iron,  13 
Magneto-electric  induction,  law  of,  11 
Magneto-exploder,  operation  of,  42 
Magneto-motive  of  force,  definition  of, 

13 
Main  switchboard:   general    description 

of,  245 ;   panels  of,  245 ;    instruments 

of  each  panel,  245 
Main  switches :  current  density  in,  263  ; 

general  construction  of,  262 
Main  and  tail  haulage,  description  of, 

306 
Marine    boiler,    89;    Messrs.   Davey    & 

Paxman's  modification  for  land  use, 

89 

Marvin  electric  percussion  drill,  descrip- 
tion of,  333 
Mavor   &  Coulson's    concentric    cables, 

223  ;  junction  boxes  for,  223 


Mavor  &  Coulson's  fuse  distribution 
boxes  for  their  concentric  cables,  259 

Meaning  of  the  term  "  pressure,"  4 

Measurement  of  work :  in  alternating 
currents,  9 ;  in  electric  circuits,  8 ; 
pressure  employed  in,  9 

Measuring  instruments  for  main  and 
sub-station  switchboards,  259 

Mechanical  stokers :  Babcock  chain,  de- 
scription of,  104  ;  description  of,  104  ; 
forms  of,  103 

Mechanism  of  arc  lamps,  47 ;  the 
Brockie-Pell  brake,  47 

Mercury  bichromate  cell,  29 

Mercury  bichromate  cells,  hints  on  re- 
cuperating, 353 

Methods  of  burning  low-grade  fuels, 
106 

Methods  of  burning  very  fine  coal-dust, 
107 

Methods  of  distribution  of  electricity, 
235 

Methods  of  producing  electrical  pressure, 
10 

Methods  of  varying  speed  of  electric 
motors,  282 

Micanite,  the  use  of,  in  commutators, 
174  ;  description  of,  175 

Mining  electric  locomotives,  description 
of,  346 ;  operation  of,  346 

Modern  drum  armature,  construction  of, 
173 

Molecular  magnets,  behaviour  of,  in 
E wing's  theory,  12 

Molecular  theory,  1 

Molecule,  the,  1 

Mond  gas :  calorific  value  of,  154 ;  descrip- 
tion of,  153 

Motive  column,  explanation  of,  104, 
340 

Motors:  best  for  driving  centrifugal 
pumps,  292;  best  form  for  driving 
endless  rope  haulage,  305 ;  best  forms 
for  main  and  tail  haulage,  306;  for 
coal-cutting  machines,  329 ;  for  driving 
fans,  339  for  driving  ram  pumps, 
295 

Motor  with  commutating  poles,  285 

Motor  generator  for  supplying  current 
for  arc  lamps,  advantages  of,  60 

Motor  generators:  description  of,  198; 
generators  with  two-  and  three-phase 
currents,  199 

Moving  coil  measuring  instruments, 
description  of,  260 

Multiple-cylinder  engines,  reason  for,  121 

Multiple  magnetic  circuits,  14;  in 
dynamo-machines,  14 

Multipolar  continuous  -  current  gene- 
rators, description  of,  172 


INDEX 


N 


National  Gas  Co.'s  oil  engine,  description 

of,  164 

Natural  induced  and  forced  draught,  104 
Negatively  electrified  corpuscles,  2 
Nernst  lamp :   compensating  resistance 
&in,  description  of,  78;  and  reason  for  use 
of,  78;  connections  of ,  78 ;  distribution 
of  light  with  different  globes,  79  ;  effi- 
ciency of,  79 ;  general  description  of, 
76;  the  "glower,"  description  of,  76; 
operation  of,  77 ;  time  taken  in  heat- 
ing, 78 ;  the  heating  coil,  78 ;  precau- 
tions in  connecting,  79 
"Nicking"   with  electric    drilling    ma- 
chines, 335 

Niclausse  water-tube  boiler,  description 
of,  96 


Oechelhausen  gas  engine,  description  of, 
161 

Ohm,  the,  5 

Ohm's  law  :  of  electric  current,  4  ;  with 
opposing  pressures,  4 

Oil-cooled  transformers,  206 

Oil  engine  and  the  petrol  engine,  163 

Oil  engine,  spraying  apparatus  for,  163 

Oil-engine  governors  and  gas-engine 
governors,  165 

Oil  engines :  different  methods  of  va- 
porizing, 163 ;  explained,  163 ;  igni- 
tion by  electric  spark,  disadvantages 
of,  166 ;  ignition  problem,  165 ;  use  of 
water  jet  with,  164,  vaporizers  for, 
163 

Oil  filters,  117 ;  Wells',  117 

Old  wire  ropes,  objections  to  the  use  of, 
211 

One  ohm,  lengths  of  copper  wires  in,  5 

Open  arc,  45 

Open-arc  lamp,  forms  in  which  carbons 
burn,  46 

Open  circuit,  6 

Operating  board  for  high  tension  switches, 
253 

Osram  lamp,  efficiency  and  sizes  made, 
71 

Otto  cycle,  description  of,  157 

Output  of  alternators,  191 

Overhead  conductors,  212  ;  limiting  sizes 
of,  212 ;  solid  or  stranded,  212 

Overhead  rope  railways,  description  of, 
309 


Paper-covered  cables  with  bitumen  tubes, 
219 

Paper-  and  yarn-covered  cables  :  advan- 
tages of,  219;  construction  of,  219; 
insulation  of,  219;  lead  tube,  use  of, 
219  ;  protection  from  moisture,  220 

Parallel  circuits,  definition  of,  8 

Parallel  or  bus-bar  system,  description 
of,  246 

Parallel-series  circuits,  definition  of,  8 

Paralleling  compound  continuous-cur- 
rent machines,  explanation  of,  250 

Parsons  apparatus  for  high  vacua,  146 

Parsons  steam  turbine,  government  of, 
142 

Path  for  electric  current  inside  the 
battery,  10 

Pelton  water-wheel,  description  of,  88 

Poles  for  supporting  insulators,  215 

Portable  accumulator  electric  lamps, 
charging  from  motor  generator,  81 

Portable  electric  lamps,  80;  Sir  Joseph 
Swan's  accumulator  lamp,  81;  the 
Sussmann  accumulator  lamp,  81 ; 
the  Sussmann  accumulator  lamp, 
charging,  81 ;  with  flexible  cables,  81 

Positively  electrified  corpuscles,  2 

Power  factor,  proportional  to  cos.  of 
angle  of  lag.,  20 ;  usual  standard  taken, 
226 

Power  for  lifting  trams  and  minerals, 
303 

Power  in  motor  for  single  drum  haulage, 
305 

Power  required :  for  driving  air  com- 
pressors, 345  ;  for  endless  rope  haulage, 
302;  and  calculation  for,  303  ;  for  fans, 
explanation  of,  340 ;  for  lifting  water, 
297  ;  for  main  and  tail  haulage,  calcu- 
lation of,  307 ;  for  moving  air,  calcula- 
tion of,  340;  for  overcoming  friction, 
caution  in  applying  rule,  299;  for 
overcoming  friction  of  waggons,  303; 
for  overcoming  the  friction  of  water 
in  pipes,  298  ;  for  single  drum  haulage, 
calculation  for,  305 ;  in  motors  driving 
pumps,  calculation  for,  299  ;  in  motor 
for  driving  endless  rope  haulage,  304 ; 
in  motor  for  driving  fans,  341 ;  in 
motor  for  three-phase  haulage,  308 

Pressure :  employed  with  three-phase 
currents,  243  ;  of  machines  at  the  bus 
bars,  248 ;  and  control  of,  248;  variation 
of,  with  current,  248 ;  produced  by 
belt  rubbing  a  pulley,  10 ;  by  chemical 
action,  10 ;  by  friction  of  steam,  10;  pro- 
duced in  thunder  clouds,  10 ;  required 


382 


INDEX 


for  electrolysis,  11 ;  to  be  employed  in 

Ohm's  law,  6 

Primary  galvanic  batteries,  10 
Producer  and  blast  furnace  gas,  quantity 

required  for  B.H.P.  per  hour,  155 
Producer  and  kindred  gas,  152 
Producer  gas :   calorific  value  of,   153 ; 

description  of,  153 

Prof.  Swing's  theory  of  magnetism,  11 
Prof.  Kapp's  measurements  of  magnetic 

reluctance,  13 

Propeller  fan,  description  of,  337 
Properties :  of  the  ether,  2  ;  of  magnets, 

12 ;  of  red  rays,  2 ;  of  violet  rays,  2 ; 

of  yellow  rays,  2 
Protection :   from  shock  with  overhead 

conductors,  215;     from    mischief    of 

overhead  conductors,  215 
Pushers  for  shaft  signals,  description  of, 

34 


Q 


Quadruple-expansion  engine,  description 

of,  121 

Quadruple-pole  switches,  264 
Quantity  represented  by  H.,  13;  by  B., 

13 
Quick  starting  of  motors,  reasons  against, 

278 


Rainbow,  2 

Bam  pumps :  description  of,  292 ;  speed 
of,  292 ;  with  Gutermuth  valves,  295 ; 
with  variable  stroke,  296 

Bateau  turbine,  arrangement  of,  with 
thermal  storage,  145 

Bate  of  doing  work  in  electric  circuits,  8 

Beciprocating  steam  engines,  119  ;  high 
speed  and  low  speed,  distinction  be- 
tween, 119 

Becuperating  cells  by  aid  of  current, 
objections  to,  353 

Belays  for  mining  signals,  description  of, 
36;  reason  for  use  of,  with  mining 
signals,  36 

Besearches  of  Hertz,  3 

Besistance :  application  of  the  laws  to  the 
insulation  of  cables,  5;  explanation 
of,  5;  for  varying  speed  of  series 
motor,  caution  about,  283  ;  the  laws  of, 
in  any  substance,  5 ;  variation  of,  with 
temperature,  5 ;  with  continuous-cur- 
rent arc  lamps,  how  calculated,  62 

Beversible  booster :  description  of,  197 ; 
efficiency  of,  197 


Bevolving  field    alternator:  description 

of,  188  ;  exciter  for,  189 
Biedler  ram  pump :  description  of,  293  ; 

speed  of,  293 ,  efficiency  of,  293 
Bing     continuous  -  current      armature, 

description  of,  172 

Binging  keys  for  shaft  signals,  descrip- 
tion of,  34 
Botary  converter :  description  of,   199; 

ratios  of  conversion  from  continuous 

to  alternating  currents,  199 
Botary  converters  in  sub-stations,  200 
Botary  heading  machine :  description  of, 

328  ;  operation  of,  328 
Bubber  insulators  for  engine-road  signals, 

30 
Bules  for  testing,  351 


S 


Schwartzkopff  method  of  burning  coal- 
dust,  108 

Scrubber  for  suction  producer  gas,  154 

Secondary  batteries,  193 ;  employment  of, 
in  generating  stations,  196  ;  forms  of, 
on  the  market,  193;  leakage  from, 
195 ;  precautions  in  using,  196 ;  pres- 
sures of,  195  ;  rated  discharge  of,  195  ; 
usual  arrangement  of,  195 

Self-excitation  of  continuous  -  current 
machines,  explanation  of,  175 

Self -ex  citing  continuous  -  current  ma- 
chines, 175 

Self-government  of  motors,  explanation 
of,  275 

Separately  excited  continuous  -  current 
machines,  176 

Series  circuit,  definition  of,  7 

Series,  connection  of  battery  cells  in,  7 

Series  and  parallel  circuits,  7 

Series  -  wound  continuous  -  current  ma- 
chines :  description  of,  177 ;  use  of, 
177 

Series- wound  motor :  properties  of,  272 ; 
and  the  ram  pump,  295 

Shackle  insulators  for  terminating  engine- 
road  signal  wires,  31 

Shades  and  guards  for  incandescent 
lamps  underground,  75 

Shaft  cables :  fixing  by  cleats,  231 ;  fixing 
in  boxing,  232;  fixing  in  iron  pipes, 
233  ;  fixing  on  insulators,  231 

Shafts  of  metalliferous  mines,  descrip- 
tion of,  322 

Shaft  signals  :  general  description  of,  33  ; 
wires  employed  with,  33  ;  with  one  or 
two  batteries,  33 ;  worked  from  motor 
generators,  35 

Shell  transformer,  202 


INDEX 


383 


Shot-firing  by  electricity,  general  de- 
scription of,  41 

Shunt- wound  continuous  -  current  gene- 
rator, with  low  armature  resistance, 
180;  variation  of  pressure,  with  varying 
external  current,  179  ;  with  variable 
field  current,  179 

Shunt-wound  continuous  -  current  ma- 
chine, description  of,  178 

Shunt-wound  motor :  for  main  and  tail 
haulage,  306  ;  properties  of,  273  ;  and 
the  ram  pump,  295;  self-governing, 
275 

Siemens'  electric  rotary  drill,  332 

Siemens-Ilgner  apparatus,  use  of  fly 
wheel  in,  315 

Siemens-Ilgner  electric  winding  appara- 
tus, description  of,  314 

Simple  alternating  current,  16 

Simple  engine,  120 ;  work  done  by,  work- 
ing expansively,  122;  working  non- 
expansively,  122 

Single  and  double  break  switches,  264 

Single  and  double  throw  switches,  265 

Single-phase  current,  16 

Single-pole  switches,  263 

Single,  two-  and  three-phase  trans- 
formers, description  of,  205 

Sir  Isaac  Newton's  theory  of  light,  2 

Sirocco  fan,  description  of,  338 

Six-phase  currents,  time  interval,  17 

Sizes  of  cables  :  allowing  for  power  factor, 
226 ;  factors  controlling,  224  ;  formula 
for  calculating,  225 

Sizes  of  three-phase  cables,  formula  for 
calculating,  226  ;  two-phase  cable,  226 

Slowly  revolving  engines  with  Corliss 
valves,  120 

Sound  waves,  2 

Sources  of  current  for  mining  signals,  27 

Special  magnet  steel,  magnetic  reluct- 
ance of,  13 

Specific  heat,  9 ;  of  air,  100 ;  of  metals, 
10  ;  of  nitrogen,  101 

Specific  inductive  capacity  defined,  17 ; 
of  impregnated  paper,  17;  of  india- 
rubber,  17 

Specific  resistance :  definition  of,  5 ; 
of  insulators,  5 ;  of  iron  and  steel,  com- 
pared with  silver  and  copper,  5 

Spectrum,  2 

Spur-gearing  for  haulage  gear,  308 

Squirrel-cage  rotor  for  three-phase  motor, 
description  of,  279 

Standard  unit  of  resistance,  5 

Starting  electric  motors,  care  in,  277 

Starting-gear  for  electric  motors,  reason 

for,  275 

Starting-gear  for  motors  for  coal-cutting 
machines,  329 


Starting  three-phase  motors,  279 ;  reasons 
for  reducing  current,  280 

Stationary  transformers  :  [losses  in,  204  ; 
operation  of,  202 ;  and  power  factor, 
203 ;  proportion  between  iron  and 
copper  losses,  204  ;  self-regulation  with 
output,  203  ;  use  of,  200 

Steam  boilers,  forms  of,  88 

Steam  jacketing  cylinders,  128 

Steam  turbines,  141 ;  >and  condensing, 
146 ;  the  Parsons,  admission  of  steam 
to,  142 ;  the  Parsons,  general  descrip- 
tion of,  141 

Step-down  transformers,  202 

Step-up  transformers,  202 

Stirling  water-tube  boiler,  description  of, 
92 

Stopping  at  different  levels  in  metallifer- 
ous mine  shafts,  322 

Stranded  overhead  conductors,  disad- 
vantages of,  213 

Sub-station  switchboard :  description  of, 
254;  for  current  from  power  station, 
254 ;  for  distributing  points  in-bye, 
254 ;  for  pit  bottom,  254 

Suction  gas  producer  :  fuel  consumed  per 
B.H.P.,  155;  water  required  with, 
155 ;  advantages  claimed  for,  155 

Superheated  steam,  advantages  of  differ- 
ent degrees  of,  133 ;  thermal  con- 
ductivity of,  133 

Superheater :  Davey  &  Paxman's,  131 ; 
Tinker's,  130 

Superheaters,  forms  of,  129 

Superheating,  advantages  of,  131 ;  diffi- 
culties in  the  way  of,  132 ;  general 
description  of,  129;  quantity  of  heat 
required  for,  discussed,  130 

Supplying  lamps  from  a  machine  com- 
pounding up,  181 

Supporting  arc  lamps,  general  arrange- 
ment, 65 

Supporting  bolts  of  insulators,  iron  or 
steel  v.  wood,  214 

Supports  and  protection  for  lamps  in 
mines,  74 

Switchboard  gear  for  high  tensions  and 
extra  high  tensions,  explanation  of, 
253 

Switches  for  use  underground,  258 

Switches :  fuses  and  circuit  breakers, 
262 ;  with  carbon  break,  description  of, 
265 

Synchronizers,  description  of,  250 

Synchronizing  apparatus,  necessity  for, 
249;  with  lamps,  description  of, 
249 

Synchronizing  panel,  246 

Synchroscopes,  description  of,  250 

Systems  of  coal  working,  325 


384 


INDEX 


Tantalum  lamp,  description  !of ,  70 ;  effi- 
ciency and  life  of,  71 

Telegraph  wireless  straining  vice,  descrip- 
tion of,  31 

Telephone  exchanges :  batteries  for,  38 ; 
for  mining  work,  operation  of,  38 

Telephones  for  mines,  general  arrange- 
ment of,  37 

Telephonic  communication  on  engine 
roads :  microphone  apparatus  for  use 
on  engine  roads,  40 ;  operation  of  40 

Temperature  that  may  be  reached  in  a 
cable  on  short  circuit,  228 

Testing  by  electrostatic  capacity  of  cables, 
370 

Testing  cables  for  disconnection,  369 

Testing  for  conductive  resistance,  367 

Testing  for  disconnection :  in  alternating- 
current  armature,  361 ;  in  continuous- 
current  armature,  359 ;  in  field  coils, 
363 ;  in  mining  signals,  354 ;  in  tele- 
phones, 356;  in  three-phase  currents, 
362 ;  in  two-phase  armatures,  362 

Testing  for  insulation  of  armoured 
cables,  366;  insulation  of  continuous- 
current  armature,  364;  insulation  of 
unarmoured  cables,  367 ;  leakage  in 
mining  signals,  355 

Testing  switches,  apparatus  used  for,  371 

Testing  with  low-reading  volt  meters  and 
dry  cells,  352 

Theories  of  electricity,  1 

Thermal  insulation  of  steam  cylinders 
discussed,  128 

Thermo-electric  apparatus,  use  of  con- 
struction of,  11 

Thermo  electricity,  law  of,  11 

Thornycroft  water-tube  boiler,  descrip- 
tion of,  95 

Three-core  cables:  description  of,  224; 
reason  for  use  of,  224 

Three-phase  alternators :  mesh  connec- 
tion, 190  ;  star  connection,  190 

Three-phase  currents,  16 

Three-phase  machine:  difference  in 
pressures  and  currents  with  mesh  and 
star  connection,  192 ;  measurement  of 
output  of,  198 

Three-phase  motor  compared  with  shunt 
motor,  280  ;  description  of,  278 

Three-phase  motor:  for  main  and  tail 
haulage,  306 ;  objections  to,  for  driving 
fan,  339 ;  operation  of,  280 ;  revolving 
field  in,  280 

Three-phase  motors  for  coal-cutting  ma- 
chines, advantages  and  disadvantages 
of,  330 


expansion  governors  com- 


Three-wire  system,  explanation  of,  238  ; 
with  electrically  driven  balancer, 
240;  with  one  generator,  239;  with 
steam-driven  balancer,  240;  with  two 
generators,  239 

Throttle  and  exp 
pared,  126 

Throttle  governor,  description  of,  126 

Tightening  engine-road  signals  wires, 
31 

Time-limit  circuit  breakers :  construction 
of,  269  ;  forms  of,  269  ;  reason  for,  269 

Trafalgar  Colliery  electric  pumping  plant, 
description  of,  286 

Transmitting  power  from  electric  motor 
to  haulage  gear,  308 

Triple  concentric  cables,  description  of, 
223 

Triple-expansion  engine,  description  of, 
121 

Triple-petticoat  insulators,  213 

Triple-pole  switches,  264 

Trouble  with  brush-holders,  general  notes 
on,  365 

Troubles  with  ring  and  early  drum  arma- 
tures, 173 

Turbines  using  exhaust  steam,  145 

Turning  up  commutators,  faults  caused 
by,  361 ;  the  best  plan  to  adopt,  361 ; 
precautions  in,  360 

Twin  arc  lamps,  description  and  employ- 
ment of,  53 

Two-cycle  gas  engine,  operation  of,  161 

Two-phase  cables,  construction  of,  224 

Two-phase  currents,  16 ;  explained,  16 

Two-phase  machine,  measurement  of 
output  of,  191 

Two-  and  three-phase  alternators :  con- 
nections of,  with  drum  armature,  190 ; 
description  of,  189 

Two-  and  three-phase  circuits,  16 

Two-  and  three-phase  machines,  propor- 
tion of  output  as  compared  with  single 
phase,  192 

Two-wire  system,  explanation  and  use  of, 
236,  238 


Values  of  power  factors,  20 

Variation  in  effect  of  heat  upon  resist- 
ance of  conductors,  227 

Variation  in  magnetic  reluctance  of 
different  bodies,  13 

Variation  of  pressure  :  at  ends  of  cables, 
210  ;  in  cables,  effect  on  lamps,  211 

Variation  of  speed  of  fan,  339 

Varying  speed  of  series  motor :  by  shunt- 
ing field  coils,  284;  by  varying  pres- 
sure, 283 


INDEX 


385 


Varying  speed  of  shunt-motor  by  varying 
field  current,  284 ;  by  varying  field 
currents,  limits  of,  285 ;  by  varying 
field  current  with  sparking,  285  ;  with 
commutating  poles,  285 

Varying  speed  of  three-phase  motor,  286 

Ventilation  compared  with  electric  dis- 
tribution, 339 

Virtual  or  effective  currents,  use  of,  15 

Virtual  or  effective  pressures,  use  of,  13  ; 
definition  of,  15 

Virtual  or  effective  currents,  definition 
of,  15 

Volt,  the,  5 

Volt-meters  and  ampere  meters,  different 
forms  of,  259 

Vulcanized  rubber  insulated  cables,  con- 
struction of,  216 


W 


Water-cooled  transformers,  206  ;  energy 
delivered  by,  85 

Water-power :  the  cost  of,  84  ;  conditions 
necessary  for  the  use  of,  84 ;  when  it 
is  economical,  84  ;  measurement  of,  85 

Water  softeners :  general  description  of, 
116  ;  reason  for,  116  ;  Royle's,  116 

Water-tube  boilers,  forms  of,  91 

Water-turbines,  description  of,  87 

Water-wheels,  description  of,  87 

Watt,  the,  9 

Watts  in  1  H.P.,  9 

Wave  theory,  1 

Westinghouse  drum  armature  alternator, 
187 

Westinghouse  Co.'s  high-tension  oil- 
immersed  switch,  253 


Westinghouse  Steam  Turbines,  descrip- 
tion of,  145 
Westinghouse  system  of  electric  winding, 

description  of,  317 

Whiting  system  of  winding  for  metalli- 
ferous mines,  323 
Willans'  engine,  120 

Willans-Parsons  steam  turbine,  descrip- 
tion of,  142 
Willis'     engine    road-indicator    signal, 

description  of,  24 

Winding  by  electricity,  309;   the  Lah- 
meyer  Co.'s  arrangement,  description 
of,  313 
Winding  of  continuous-current  armature 

coils,  172 

Winding-problem  discussed,  310 
Winding :  sources  of  waste  in,  310 ;  power 
required  at  starting,  311;  power  re- 
quired for  acceleration,  311;  power 
stored  in  descending  cage,  311;  in  stable- 
pits,  312 ;  by  shunt-  and  series- wound 
motors,  directly  connected,  312;  in 
metalliferous  mines,  322 ;  by  three- 
phase  motors,  directly  connected,  312 ; 
waste  with  directly  connected  motors, 
312 ;  variation  of  pressure  with  directly 
connected  motors,  313;  variation  of 
pressure  at  Preussen  II  Colliery,  313 
Wireless  telegraphy,  3;  unsuitable  for 

mines,  40 
Wires  and  cables  for  connecting  to  lamps, 

228  ;  protection  of,  228 
Wirt  Co.'s  copper-brush,  182 
Work  done  in  electric  circuits,  8 
Working  steam-engines  expansively,  120 
Working  units  to  full  power,  208 
Worm  gearing  for  haulage  gear,  308 
Wound-rotor  for  three-phase  motor,  279 


THE  END 


2c 


PRINTED  BY 

WILLIAM  CLOWES  AND  SONS,   LIMITED, 
LONDON  AND  BECCLES. 


u 

THIS 


UNIVERSITY 


OCT  22  '  - 
FEB  7    1942 


50m-7/16 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


